@PUNKEY,
I say sex is an integral part of any relationship and if their not able to agree on something as basic as oral sex, then it does not auger well for their future.
So why prolong the agony.
It's better than some of the other suggestions I've heard, "Put a bullet in his head"
or your idea "Tell her many men go see a prostitute" that's a nice basis for a loving relationship.
Did you mean for him to actually go & see a hooker or threaten his wife that he will if necessary, because either way, it sounds nasty to me.
So if they cant find common ground on this or he IS willing to accept it as it is. Then YES they should split before they make each others life a misery.
P.S. .....MARRIAGE is a relationship, no more, no less.
Not being MARRIED does not make it any easier for people to split up.