Just reading that, I'd have to say size Small.cycljunkie said:
Of course, it all depends on her legs, torso and arms in terms of proportions. She sounds like a classical "tweener" and could go either way. Based on her height alone, she falls right on the edge for either being the tallest height for the size S or the smallest height for the size M. Compare the frame measurements of the S and M for the Big Mama to the Canzo and Supercaliber she had. If the Mama size M is similar, you don't want to repeat the poor fit you had on those bikes. So step down a size and enjoy.
Any chance for her to test/demo a Big Mama in either size in your area?
BB