I think you just don't get it dude, but that's okay. It seems like a very male perspective to suggest it all just comes down to being interested or not, and if they're interested, why wouldn't they do it... Maybe your wife is like that but I don't think most are. You didn't offer this guy any useful advice, just told him his wife needs to be interested, as if he should just wait around until she decides to do something on her own or give up.
The whole point of my previous example is that what applies to sex in general, applies even more so to sex with someone else. She may be interested (fact she already went so far suggests she is) but was unable to go all the way due to conflicting feelings, which could be fear, guilt, shame, lack of confidence, etc. She may just need encouragement and support to work through that or simply further opportunities to ease into things.
I just don't think this is as simple as, you just tell the wife you want to see her fuck another dude and she will do it or not, depending on her interest.