<human>: Can you provide a brief overview of the text?

I'm posting about my friend's situation. Is a tough one, and I am often at a loss on how to advise him. 

He's been married about ten years. His wife developed postpartum depression after their first child was born nine years ago and has not recovered. Like, at all. She literally does nothing but watch tv and sleep, though she does go out with friends and is interested in sex, which is perplexing. I don't know her well, but the times I have spoken with her, she seemed"not there". Very odd, and kind of scary. 

He has tried everything to help his wife. Therapy, marriage counseling,  support groups, medication. ..everything. nothing works. She is a poor mother and quite neglectful, which is the worst part by far. 

He says he no longer loves her, and that he doesn't feel like he has a family. He spends as much time as possible with his kids and she is never there. There are no family dinners, outings, or any kind of togetherness at all. There is no support, intimacy, or partnership in the marriage. He's miserable. 

He wants to leave, but doesn't feel like he can. He compares it to leaving someone who is seriously physically sick. .. you wouldn't leave if your partner had cancer, right? I can definitely see his point. 

On the other hand, I worry that the misery he's living with will last the rest of his life. He's a great guy and deserves to be happy, but this situation with his wife has him trapped.  He has mentioned wanting to have an affair to ease the loneliness he feels, but I advise him against that because it will only complicate things for him emotionally and potentially ruin his kids' lives. 

What advise would you give to someone in this situation?
<bot>: Friend trapped in a miserable marriage with a mentally ill wife and doesn't feel like he can leave. His future looks grim if he stays, but the guilt of leaving keeps him with her. What to say to help?