Don't Presume
As an old married guy with daughters, don't make the mistake of "thinking that you know best" for her. Let her show you what bikes she likes, then help narrow it down based on the good suggestions that have been given in previous posts. Any bike is going to start out exiciting, but one that she picked because she likes the look and feel, color, etc. will stay interesting a lot longer. Expose her to a variety of styles makes and brands and take the time to figure out what turns her on.
An example; despite my youngest daughters exposure to KTM, Yamaha (my 990 Adventure and FZ-1 and FZ600 as well as a KLR 650), when she last went to a multi-brand dealer with me her picks were a Suzuki Gladius, a Kawi' Ninja 650, and a yellow FZ-9 (in that order). I tried to steer her toward a KTM RC-390 but it didn't turn her crank. She liked all the "Ninjas".
If a machine doesn't "grab you"there is no point having it, as it will just get traded away and cost you more in the end.