I think she would wear a misses small.Get a multi size pattern,trace off the small size with some wax paper,pin it together and try it on her.Make or note any adjustments you need to make.I'm sure some of the other ladies have some ideas as well.When i use wax paper,i sometimes have to tape 2 pieces together to get a piece big enough,Then i use either a ball point pen that no longer writes or my tracing wheel to trace the size with.I also buy my wax paper at the .99cent store.HTH
Kathi

FWIW, pattern catalogs show how to measure for sizes - usually in the back of the book....

Here is a link that might be useful: McCalls - choose your size