Yes her height will be exacerbating the footrest location problem. She will feel like she is being pulled off the back of the bike every time you accelerate or change gears up.
There was a kit around to extend the footrests down a little, but unfortunately the mounts onto the frame aren't all that solid, and extending the bracket applies more leverage to those mounts.
But the bracket extenders might make your girl feel a little more secure. It would allow her to brace against accelerative forces a bit more.
But - just by way of illustration - my wife has done many miles on the back of my bikes. The old bevel 900SS, a later '90s SS, and our dual-purpose Elefant and others.
Whereas she enjoyed many a run into the outback on the Elefant - long distances up the notorious Oodnadatta track and out to Arkaroola - she didn't want to go beyond the limits of our small suburb on the back of the 1098.
That is how shit it is on the back of them for a full-sized passenger. (She is reasonably tall for a girl).
So I guess you are down to working out how important this girl is to you, and how important it is that she shares your bike riding. :lovebirds:
Best suggestions we can offer..