Is the fact that she is 18 matter? What I mean by that is...are you implying that she is slight, or has small hands, or is generally frail? Or is she athletic and as physically capable as the average guy?
Would a revolver be "best" because she cannot figure out the awesome complexity of a semi-automatic pistol?
I'm just wondering if you are bowing to some stereotypes of women shooters or know something about her that we should know to better answer your question.
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face