oh dear this is a tricky one, i think it would depend on how much he loves her doesnt it? obviously there must be something wrong with their marriage for her to do this in the first place, i think that they would need to sit down together and have a really long talk about their marriage and why its gone so wrong. i feel sorry for your friend he must be feeling very hurt and betrayed at the moment, thank god hes got a good friend like you to support him well done. i think if he did forgive her then later on down the line it would keep cropping up during any arguments etc and i doubt he would get the thoughts out of his head, like i said earlier this is definately a tricky one.
i wish your friend and his wife all the best , good luck