Well, tell her it's better to save than spend.
But personally, I'd tell her the following:
1) It takes at least 6 hours for a full charge on that thing even with the supplied charger, and it won't even charge sometimes when you try to use it while it's plugged in. In other words, the thing is a battery hog. She still gets 10 hours of continuous use out of it though.
2) It's generally not too useful if your house is full of Windows computers.
3) It'll cost her more money to buy cases and accessories for it, and trust me, she will be tempted to, so it's not just half a K. It'll be more like three quarter K by the time she's done with it. She's gonna have to layer about $50 - $100 worth of app money on top as well, over time.
4) It doesn't generally offer her a better computing experience than a laptop, even though it may seem to be a more convenient device to use.
5) She will find it hard to look at other screens after exposing herself to the new iPad's screen over an extended period of time. That's a sincere warning.
6) In the case that she decides it's wasting too much of her time (and trust me, it can be addicting), she's out at least half a grand for a paperweight.
7) In the case she is truly addicted to it, it'll take up the majority of her time, and she'll have less time to do other things.
8) If you can't stop her even at this point, imply she has 14 days to return it.
9) If she ends up enjoying it, God helps you, your mission has failed.