I am about her size, and is challenging to find a cross frame that fits! You might want to check out Ridley, as they have some frames in extra small sizes. She will probably need to have smaller wheels also. I use 26 inch wheels on my Terry Valkyrie. You might be able to get just the frame from Terry, if you want to build the bike yourself. I got the complete bike from them and my BF built it up. Terry's prices have gotten quite high, however.

Ridley Crossbow

Valkyrie-Tour