It's probably going to be extremely difficult for him to ever trust her again. But if he thinks they have a chance to mend it, well, that's their choice and no one else can really make it for them. :-(
True, my whole thing in it is he's coming to me looking for some advice and I'm having a hard time trying to tell him what he wants to hear.
I've been on both sides of the fence.. It is possible to get passed it, and the anger/sadness/lack of trust does subside, but not for quite a while.. This isn't something that's going to go away in a month or two, it'll be years, and there will always be that nagging 'what if', if he can't put his trust in the fact she won't do it again, then I would tell him to not even bother trying.. Because even if you trust they won't do it again, there's still a certain level of broken trust you feel.. Or maybe betrayed would be the correct term.
You shouldn't tell him what he wants to hear, you should tell him the truth. The above, is the truth.