I would try e-6000, it works pretty well on crystals, I use it all the time on costumes (dance and skate). I would test it on plastic first though.
Also, if she is going to buy crystals (swarovski) this is the cheapest
site I have found. http://www.dreamtimecreations.com/
Make sure she buys flatback and not hotfix...Hotfix are
more expensive and not needed with glue.
Good luck! I LOVE BLING!