I know it's been said already but I'd reiterate that a 14" On-One would probably be too long imo. I'm about 5'4" and wouldn't want the TT any longer, in fact I'd prefer it shorter!
Rigid forks are a good plan if she's not really riding trails and I would recommend a 1x9 set up if a lot of ups and downs aren't being covered. I've run 1x6 (hope ss hub) before and had no probs with chain drop, etc. and the simplicity is nice.
I also used to have a 15" 853 Rock Lobster and although shorter than the Inbred, I still reckon it's still likely to be on the longside for your lady. I'd try one of the smaller Konas or perhaps a dirty Jo frame - the xs weighs in at about 1.6kg with an effective TT of 20.5"... so could be ideal.
One of those little frames with some nice carbon forks and a light wheelset (something with 717 rims or similar will make a massive difference!) would be lovely and light and not too much cash at all.
It is worth getting her a light build as weight makes a huge difference to us girls and it might be all she needs to get hooked... it's got to be worth a try anyway?!
Hope that's helpful! MM