I think it should be taken into consideration that on-one bikes have very long top tubes, and I don’t really think they’re suited to women.
I’m 5’4″ (on a good day), and have rode a 16″ Kona, a 15.5″ Dialled Prince Albert, a 15.5″ Dialled Love/Hate (which was probably a tad too long), and a small Santa Cruz Heckler.
The Santa Cruz Heckler is my favourite – as I like a really short bike for steep stuff, but a friend the same height as me preferred the Love/Hate (my least favourite). <– This makes me think you should just make this person try a few types of bikes before building.