So many variables here that it is hard to answer...of course a bow set at 40lbs could kill a deer.....if you are close enough to do so without losing trajectory. In my opinion if she is comfortable with 50lbs why not go that? I wouldnt have my g/f shooting less than she was capable first of all and secondly, I surely wouldnt want to take the chance of having a no kill shot. This is her first time bowhunting make it an experience that she will remember....a good solid KILL.

Good Luck to you and her!