I agree, not everyone is meant for three wheels, and I'd hate to put someone on something they're uncomfortable riding.
Take it slow, ride with her a time or two putting around the yard, and let her get a feel for turning and stopping and all that good stuff.
Rule #1: Wear your helmets!
This is my trike. There are many like it but this one is mine. My trike is my best friend, and it is my life. I must master it as I must master myself. My trike without me is useless. Without my trike, I am useless. I must ride my trike true. My trike and I know that what counts is not the amount of miles we ride, but the time we ride together. My trike is human in it's own ways, therefore I will learn it as my brother. I will learn it's weaknesses, it's strengths, it's parts, and it's soul.