I am an old school HP48 user...once you go RPN, you don't like the other way. The last one I had I wore out the buttons. Now I just have an HP48 emulator on my phone. Beats the crap out of the regular calculator.
Every reasonably modern smartphone has scientific calculator capabilities built-in.
For a standalone calculator, unless she really needs graphic capabilities (in which case you're better off using a computer), then the good ol' TI-30 series does just fine, at a cost of 10 bucks from Wal-Mart, etc. You can also find them for 7 bucks or less, shipped, through eBay.