Zuggie, I suggest perhaps mentioning to your opponent that this is generally accepted as illegal. Maybe they just didn't know that. I'm not sure if Fencer would put the game back a move so that they can then make a legal move, but at least that person will now know it was wrong.
I, too, hope that someday Fencer will program the backgammon games to follow this rule. In the mean time, we just have to live with it!