I have a friend who stayed married for the kids. Wife treats him like shit. Kids are now rotten because they treat everyone the way mom treats dad. Dad tries to teach them the way mom treats him is not the way they should treat others. Not clear it's effective.
Kids take cues from those around them not just verbally, but emotionally and actively.
If your friend stayed married and responds to being mistreated passively, that isn't enough (for him or the kids).
Funny thing... my ex says unkind things about my partner. Kids pick up on this and it has actually strengthened their relationship with her and has harmed their relationship with their mother. Kids aren't dumb, they know when adults are being unkind and irrational.