If you cannot get her to wear a dress, maybe find a seamstress who can make her a pant suit in the same fabric as your BM dresses? I think there are some designers out there too who offer dresses AND pants as well...if you choose the color and
fabric, each gal can choose her design...and for this friend, in particular, it could be pants. If you put the other gals in long, two piece dresses, she could wear full-cut pants and blend in nicely. Silk shantung would work beautifully for
this...you could buy some in whatever color you like and gice enough yardage to each girl so they can have their dresses or paint suits made to order...I have a friend who did this (one of her BM's was her FSIL who was 15 years older than all the other
girls) and each girl had a dress that fit their personal sense of style and body type, but they all coordinated.
On the otherhand, if she TRULY is your friend, she'll realize that your wedding is not about her! Surely she can don a dress for a few hours! Try talking to her about it and see if there is ANY flexibility.
Not sure what your colors are, but here is an example of choosing a pant suit for your MOH that could coordnate in with other BMs in dresses:

The other girls could wear something like this: