I have a niece who was very into them (I think I got her hooked when she saw my collection, actually :p) This was during the height of the G3's, and I fondly remember a lot of fun times playing with her, she loved 'borrowing' the big brother ponies to play the dads of the (extensive) pony families. She was about 12 I think, around the time the G3.5's hit...and she absolutely hated those (I wasn't too fond of them either :p) and that kind of killed it for her. I'm surprised she was into them for as long as she was. She's off in college now and entrusted me with her collection, to keep just in case, someday she'd like to collect them as an adult, or pass to a child of her own.
Everyone changes their mind about things, and whether or not she's sincerely not wanting to play with them now, or keep them on display, some day she just might. I know my biggest regret as a kid was not keeping a lot of my original pony stuff (had a SE dream castle I REALLY wish I'd held onto now!) I had begun to feel silly for having them, it wasn't until I got a bit older that I decided I didn't care, and didn't want to avoid something I liked just because it might be abnormal to some. Not sure why I ever felt silly though, everyone in my life now is supportive and has a pony or two of their own!
But as everyone's said, best thing is to just hold onto them, even if she never really feels the desire to have them again, least you'll have a collection to remember when she did.