Yep, have her spread her arms out at shoulder height, palms forward, fingers extended and measure her wingspan in total inches. Take that number and divide by 2.5 and you'll have a good rough/roundabout guess.

My best advice and experience here would be to use that number as a STARTING point.... it might be dead on.. but you may have to shorten it a bit as a female's arm is built differnetly than the male arm.

Whatever bow you get her... make sure the draw length is adjustable at least +/- one or two inches on either side of the draw you measure... that will give you some room to play with so she can find exactly what is most comfortable for her.

Good luck! And congrats on getting the Missus on board! I shoot with mine and couldn't imagine shooting without her!