Pawn shops are a good source for used bows, or Craig's list. Early compound bows had a 50% let off, meaning at full draw if it was set up for a 60lb pull weight, you would only be holding 30lbs. The archer still has to overcome the initial 60lb pull before the bow cams over to reduce the weight.
Modern compound bows go up to 70% let off now. Maybe even higher in the upper end bows.
Any archery shop can raise and lower the pull weight to make it comfortable for any size person.
A 40lb pull bow is legal for deer in Oklahoma.
A bow set at that weight with a 50% let off is real comfortable to shoot.
So, if she is looking for a plinking bow, get what you can afford, take it to a good bow shop, have it tuned, and have fun with it.
As time goes on one can look at higher pull weights, and higher quality equipment.
Hope this snswers some of your questions.