Oh Allison! Please give her a talking to! We all know that MANY non-greeks have negative views of greeks.
Maybe this guy has only had contact wth the soroity members who do happen to be "sterotypical"! How could he possibly know all the different types in all the different houses at Arizona (b/c there are a lot!)? Heck, in my chapter of about 80 girls we had intellectuals, jocks, partiers, rich, poor, fat, skinny, short, tall, quiet, loud - you name it, we probably had it! And most of the houses on campus had diverse girls (although personally I think my chapter took the prize for MOST diverse!

). However, there was a chapter or two who did have quite a lot of the sterotypical "rich, blonde, snotty" girls, but that was by far the exception to the norm!
Remind your sister that she shouldn't judge until she sees for herself! And even if she decides not to pledge, she might make some new, great friends from her Rho Chi group!!!