I am taking my sister boarding tomorrow and I have 3 boards to give her: an Alp 164, Amp 6.0, and Liquid Arc 164. She weighs 112lbs.
We flexed the boards. The Alp, which has only been used once, is the stiffest. The others are used more, and they both were a bit softer, about the same between them.
I think the correct board would be the Amp... it's the shortest, but still almost as tall as she is.
She'll probably be using softboots.