It took me awhile to be comfortablee with the idea of the person i was with going out with another guy. I was way caught up in all the potential bad things that I was blind to anything good. I had to stop myself and realize the good side.
For one, I may like the guy and we could hang out. It would also give my wife time to do some stuff that I don't like to do. Plus I really started to think about compersion and I realized I could be happy for her if she was happy dating another guy.
So my only advise is to see if he can try to think about any positive sides to this.