Unfortunately this is a decision you will have to make, as this is not typical as per etiquette standards for bridesmaids. When she accepted the position, she should have also accepted your choice in attire for the day. If you want to allow her choices, then you are not following the norm. This isn't improper or "a bad thing", but just not per the norm so I can't really advise. It is just so personal.
One note though, if you agree that she is to wear a suit similar to the groomsmen, she may feel awkward. So, there is that too.
Just another thought, there is no rule that states that any of the women must wear dresses. Perhaps all of your attendants could wear dressy pant suits.
Bridesemaid wants to wear pants
