I know quite a few Rileys, but they're all boys. I can't say I like this on a girl, and if I heard of a girl
Riley I'd probably assume it had an alternate spelling like
Rylie or something. If you want to use
Riley on a girl though, I don't think it's impossible. Lots of girls are being names more 'boyish' names (and
Riley has the more girlish "ley" ending), so it won't be seen as weird. And the other spellings might just make people ask her how to spell it, instead of spelling it wrong.