HK, my advice: Find out how age comes into play. It could be that she is absolutely not allowed to rush at all, or only under certain restrictions. That would be useful to know.
Since she's admitted that she's considering it for social reasons, I'd just suggest the following:
Tell her that college is for learning new things and she should ABSOLUTELY check out Greek culture to see if it is a good match for her. That she can't pledge or live in a house because of her age, but she should learn more about it, maybe be a little sister with her friend as a mentor, and possibly participate in some rush activities.
Then I'd also point her to other avenues for exploring social opportunities that might be of interest. I remember my college had a college club fair where the different groups had tables and talked about their clubs. You might point her to such a thing, or the catalog of clubs if there is one.
Finally, I'd see what offerings there are associated with the honors college (apologies if you have done this already). Again, just to point her to them for her own exploration.
I guess my thinking is that if she's looking for social connection, if you simply forbid her from all things Greek, you might just end up pushing her there (she already has a friend who she respects who is in that system). Whereas if you say "here are lots of great social things to check out and you should feel free to do so" she may just sample several and discover for herself which ones are the right fit.
As for my personal opinion of sororities, it is not very high. I understand that there are academic ones, but I was never very impressed with what I saw of Greek culture at my state school. I wouldn't have wanted to be a part of that, not for all the social life and "friends" in the world.
