I would certainly get some more time on her before putting anyone else on her, and perhaps have a trainer work with her as well, somewhere down the line.
I would guess that her issues have to do with the way your fiance is riding, or not riding her...he probably doesn't have a decent seat, holds her with his legs, and uses the reins a bit too much...all confusing and aggitating her.
I have a mare that would undoubtedly do the same thing if I put an inexperienced rider on her; she is very sensitive to weight and position, and doesn't appreciate you hanging on her mouth. But she is great for me, so I don't worry about it...atleast for now; she is still green like your mare, so I would rather not have other riders on her anyway.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."