How aggressive a rider is she? If you guys are doing fire roads, she wouldn't get her money's worth out of a KTM. However, it might be SO MUCH better that she'd go after the tougher trails.
(on a side note, I think significant others often sell their wives/girlfriends short ... by worrying too much about what they can handle, they make the gal think she can't ... but that's just a general observation I've made)
Personally, I think the KDX200 is gold for anyone tall enough to ride it. Zippy enough to have fun, smooth enough to feel comfortable, upgradeable as she progresses, and while it's heavy, it's not like the overweight little thumpers. Either that or a YZF or CRF 250 would get my vote for her. She'll get used to the height and weight, and it will undoubtably improve her riding experience. The standing-to-sitting transition on a small bike is not only uncomfortable but tiring as well. I used to work for BBR and took one of their aluminum 150s to the track along with my 200 a few times. It's kinda fun, but I felt more sketchy than I do on my fullsize bike. If I was 5'3" it'd be different.
Line up some buddies with bikes in the range you are considering and have her try them out, get a feel for whether she wants to switch to two-stroke (recommended--I LOVE IT!!) or stick with thumpers.