I don't believe there is a problem in socializing with the opposite sex. Many people feel this way, but on a DB forum may not voice it.
I don't think that she gets to treat him like she is, and sometimes even if you are the father or mother you should take side of the one who is in the right. What lesson are you teaching your child if you allow them to do wrong by someone and there is no cost to them. Support them if they fall, but they should be doing right by the one they wronged.
I stand on the technicality that if the wife is OIN's sole source of female attention that she will drive down his attractiveness due to their interaction. I know its "bad" advice.
Its one of those things about knowing how things work.
If the family is saying she did right and supporting her here with a lot of love and motivation in the direction she's moving, he's already toast.
OIN, just consider what I'm saying if you get close to being "used up", it really works and you don't have to cheat on anyone.
Also these really bad waywards, if the "see you give a damn" it makes them more resentful. I know you know what I'm saying here. They are being real bad, and saying in their evil mind "oh, and your soo good, huh?".