I think you’re unlikely to be travelling fast up or down, frankly, if she’s new to riding.
You’d be better off on a bike that can go slowly in both directions – at slow speed on trail centre stuff, a ropey hire bike isn’t necessarily a bad thing to be on. As pointed out above, if you want her to come out riding again, then you should concentrate on getting her to enjoy the ride. That’ll involve mincing both up and down in most cases.
I should point out that Mrs Udder impressed both me and friends when she came out on a mountain bike for the first time a few years ago and proceeded to ride up and down stuff very quickly indeed. But I still had to pay attention to my own speed, route choice and the rest to make sure she had a good time. She wouldn’t be Mrs Udder if I’d taken out my singlespeed that day.