Maybe they should try a trial separation to see if they truly want to be away from each other or want to give it another try to salvage what they have.It sounds like they already have their minds made up,It is hard when people you care about split up, nobody but them can really answer the question, just be there for your friend when he needs you.

oh, i beg to disagree on a certain point... some people, and i repeat, some people, do not realize how damaging it could be to others... putting their own happiness above all else... that is the point when it becomes selfish...

but to qualify my dissent on your statement... these are likely the same people who equate happiness with constant satisfaction instead of real contentment...

at some point yes it is...could be damaging. .but being in a relationship where there is no sparks..no happiness. .nothing at all..it'll still result to damage...ssuch a waste of time..

I understandwhat it means to live unhappy, boy do I ever.... But if one is going to make assumptions to base ones life and future AND those of his children (more importantly) he should do so within the realm of reality. If he is moonlighting, whoever is doing whatever with him is only doing so because he is forbidden fruit. The moment he goes 'back on the market' his appeal will be gone and prospects will be running away rather than to him.
If he isn't having an affair, he most likely has no idea how cutthroat the dating scene is recently as opposed to 7-10 years ago or however long ago he courted his significant other.
He also has to take the kids futures into account, if he doesn't they will take it out of his account. It doesn't sound like he is considering them at all if all he is worried about is the spark. He should be thankful that he has three kids by the same woman all able to live together and enjoy the hell out of that in itself. With social ruining more marriages and families everyday he should be glad that he's even in the situation he's in. Look how many kids have parents that won't be coming home from war, how many kids commit suicide because they were bullied or murdered by mass shooters.
Spark is just a very brief part of a relationship that long and complex and if you listen to the eighty somethings who are still alive and still in love through a lifetimes worth of trials and tribulations they never lost perspective of how lucky they were just to have eachother.
If she's screwing his best friend, he should leave her. If he screwing her best friend she should leave him. If that's not the case warm up that little violin and ask him if he's willing to give up likely well over half his paycheck house car bank accounts and his kids for a spark. I'd get him a magnesium stick and the card of a local marriage counselor myself.

1 of them is getting it somewhere else

i dunno... i've had my mom be so frustrated with my dad some years ago...and ive also heard of stories that happened before i was born of how she tried to leave... i think a lot of married couples go through phases in their marriage that changes each one of them along with the events transpiring... it can always go one way or the other...

as for happiness... i think that can evolve... depending on what one considers the fulfillment of happiness...

i like this ^^^, very much

I quote what you have said #1:
"there's nothing really wrong with his relationship" "they care for each other" "they have spent a lifetime building a life together they have 3 kids" ..... just this 3 sentences should be able to tell you that it is absolutely wrong to put anyone's own happiness above that of the spouse and the kids! Call me 'old school'.... but what is he going to tell the kids when he walks out? He should really work on the issues more. It is not broken yet......

I didn't read every response, so please forgive me if I'm repeating a suggestion. Marriage counseling seems like the most obvious place to start. The "spark"can always be rekindled, but only if they both try. If either one refuses to make any attempt, their marriage is doomed. I believe you are telling us the whole story as you know it, but are you getting the whole story? My guess is, probably not. That's not to say your friend is lying or intentionally keeping part of the story from you. That's to say that you know his side. She will probably tell a completely different version. Somewhere in the middle is where you will find the truth. Generally speaking, couples who work things out are happier in the long run than those who don't. All the best to them.

I think if is not chemistry anymore- better leave.
Kids will be better with two parents in separation and stay friends than stay without love but together.

Which model can be better for them and teach kids ???

I think the answer is easy :)

Chemistry can ebb and flow. It's sad that you recommend giving up so easily.

It's not putting your own happiness above all else, although there's nothing wrong with that either to be honest. Why should you suffer and become miserable in an attempt to keep others happy?? No one deserves that...

But what I want to say... if he stays .. both his wife and kids will feel that he isn't happy anymore. If he becomes miserable, they will suffer, just like him. Meaning .. .if he stays, EVERYONE will become unhappy and miserable in the end.
Kids are not only resilient, they are also sensitive and can often sense that something isn't right anymore.
And divorce scarring,, mwah... to be honest it taught me a heck of a lot! that I would not have understood and known had I not gone through that as a teen. And I see the same thing in my kids. Sure it ain't fun, sure they'd prefer mum and dad to stay together. But as long as mom and dad are happy, they will manage.

So ...if they really tried and it don't work (then the spark will never come back no more!) I'd advice him to leave, in a respectful way of course.

people walk away from relationships too easy without really fighting to save it. you could ask them both to sit down in front of each other and really look, ask them to remember what they find attractive about each other, what they love about each other what does the other one do that makes them laugh? etc and then get really deep, do you love knowing that they will always be there( if they stay together) for the hard times and the good?. and then ask them, do they want to watch their grandchildren grow up together? if there is nothing wrong in the relationship and they have children then they would be silly not to get the groove back. failing that it could just be that when they actually get down to the nasty well it isn't really nasty anymore and its more of a quick jump on and of sort of thing. I don't care what anybody says if you don't have good sex in a relationship is going to get really boring.