When I started out, unfortunately I listened to somebody suggesting lighter was better, that with a lighter cue, say 17 or 18 ounces, it was easier to draw the cueball, easier to move it around, and so on. I heard that if you had a stroke, the lighter cue was the nuts. So I imagined I had a stroke and had some weight taken out of the cue to make it lighter, and thus, supposedly, easier to move the cue ball around, yada yada yada.
Can't say as I really believe all that anymore.
To some people, my 19.2 ounce cue is butt heavy, to others it's too light, yada yada.
To me, it's just right. Lighter than this or more than just a tad lighter doesn't play as well for me. Heavier might be fine, but lighter? Nah...
As for moving the ball around and so on, I really think it's in the stroke and the cue tip. Even a great big heavy thick shaft will move the cue ball easily enough if the stroke is there, and the tip is a good one, that imparts enough juice.
In my case, much lighter than 19 ounces lets my stroke go off more easily. Perhaps someone with lighter arms and so on might just prefer a lighter cue. Not sure if a person's weight has anything at all to do with it, though.
One thing to try with her though are soft shots with a minimum stroke. See how short she can shoot the cue ball. If she has a lot of trouble and is always shooting the cue ball too far, perhaps a bit lighter cue will help.
Flex