Many of us are frame builders. I do TIG frames and often very large or small ones. The usual limitation for small people and 700c wheels is toe contact with the front wheel. If she is riding a 42 now, isn't hitting toes (or doesn't mind) additional stand-over clearance should be easy. We can certainly discuss potential options. My email is below.

Also, some other builders are able to make lugs that offer unique angles and solutions so lugs are an option!