At least you recognize the situation and are taking steps to correct it. So many men in this forum don't, and the abuse is both emotional and physical. Emotional in that she's being ****** to do something that she doesn't want to, only agreeing to do so to please her husband. Physical in that there is a very real possibility that she will feel violated and physical hurt by a man who doesn't care about a relationship, but simply someone to use.
What if the roles were changed? What if the wife told her husband that if he really loved her, he'd take it up the ass by an overly endowed guy? I doubt there would be much consideration for gentleness or his reaction as he was split open and used. I think it's the same way for the wife, but I'll admit I don't know as I've never been placed in that situation - everything has been consensual, as much my idea as my husbands.
Just remember one fact. The only thing we ever truly own is our bodies. She should be allowed to determine how she uses it. The husband may suggest (something I encourage) but that's where it stops, at the suggestion.