They’re on the cusp really, as I am at 5ft 10 1/2 (the half is important!) with the 17.5″/19″ frames. I got the 17.5″ rather than the 19″ cos I prefer a slightly smaller frame with more standover, but there’s only about 15mm in the top tube between them anyway, which is a mere stem change away. The guy I got my frame from was an inch taller than me, and he just couldn’t get on with the 17.5 and found it too small, so replaced it with a 19″ and found it much better suited to his riding style and proportions.
Are they long in the leg or long in the torso, or just average?
I’d probably go for the 19″, but it would be best to get them to have a sit on both first if possible, see what feels better. My friend is 6ft 2 1/2 (again, the 1/2 is important!), and he needed the 20.5″ for sure as the 19″ wasn’t long enough in the top tube for him.