Good question ^ And I'd probably ask her if there was a reunion book being planned. Lol nah, I probably wouldnt bug her about that... But I'd subtley make sure she knew it would be a dream come true. Other than that I'd probably ask her about her life.
I would ask about all of the things that bother people on here and other boards so much and ask if she knows about all of the snark, reads it, and if it makes her feel bad. (not that I don't do some myself but I try not to ;D)
^ Hopefully. ;D For some reason, I would also ask if she had ever ridden horses and where she has traveled and visited in the US. Also, what she liked most about writing the books, why she stopped and if she misses it. Also, if she felt like she "knew" the characters after a while.
I think I would first tell her how much I still love the books even as a 20 year old. I would then probably ask her what she envisioned each girl to do when they became adults, I would probably also ask her if she thinks all the girls would remain friends as they got older.