There is a belt called a Buddy Belt. Looks like a 4" or 5" tall nylon weight lifting belt.
It has handles for your passenger on each side.
That coupled with a back rest is probably the safest route, including full gear(at least for her).
There are belts for both driver and passenger that hook together, but personally I would have my concerns with that.
Reason being, in a crash, she is going to tumble with you, as opposed to being thrown clear.
But, either way could result in serious injury, so it's a personal choice.
Good luck.