What should I look out for?
Any other advice?
As mentioned above don't spend too much as she might hate it. They are impossible to learn unless you are young and stupid. I can't do it as I'm not young. My son has managed it and can now ride successfully. He started on a 20" wheel, small and unintimidating, and has now graduated to a 29", better for covering long distances. Shin and wrist pads more than crash helmet. You tend to fall off onto your feet but the cycle may hit you in the shins and if you trip you'll put a hand out.
I'll shout if I think of anything else.
My advice? Don't get anything expensive. Unicycles are crazy hard. I suspect that most people give up very quickly.
Really, really crazy hard.