So he has not confronted his wife about her infidelity? It's tough for a man to file for a divorce where his children are concerned because 8 times out of 10 he's not going to get custody. I've been in his situation before, wanted to file for divorce but I was too scared to pull the trigger. I thought that I would lose my kids if I filed for a divorce.
She finally filed. When she did, I hired a good attorney; I was pro-active in my own defense; I took 3-4 parenting classes and brought the certificates to court with me. I never made a negative statement about her in court, my focus was 100% on my kids (to attempt to prove the mother unfit in court is a MAJOR mistake when pursuing custody).
My attorney and a few that I interviewed before I settled on Mark Hines (out of Atlanta) all told me to go out and get my own place but do not leave home because that would be considered abandonment. But when you go to court, you have your own address which means you have a place for your kids to live. The courts look at the marital residence as belonging to the mother.
I have a son and a daughter, it may be more difficult for your friend to gain custody of his daughter. If he believes he can get joint custody, or he's OK with the mother having custody, especially if he has unlimited visitation, he should file for divorce. Some people would say, go to counseling, try to work it out. If he knows that she is cheating, anytime she leaves the house without him, he won't trust her to tell him the truth about her whereabouts. It's very difficult to overcome a cheating spouse. He could start cheating himself but that never solves anything and just makes the situation worse.
Tough call, but I'd seriously consider my options and how I could spend the most time with my daughter.
Good luck to your friend, let us know how it turns out.