Yes, I think the rear is definitely too big for her. I would think she needs about a 16". Be patient, the right one will turn up.
If it's local and you can try it out, then I'd take that opportunity, maybe just to learn what that size is like. But I don't think you should buy it. FWIW, my wife and I rented a tandem bike about 3 years ago. She loved it, but we just enjoyed our 1/2 bikes for about two years until the right tandem showed up that we couldn't refuse. Take your time shopping and do some test rides.