Ugh...I've typed a big long response three times and my computer keeps freezing up! Ticked!
1...she will be tall enough, but is she "big" enough? We took my niece on a water ride that she was tall enough for, but she is soooo tiny, she nearly fell out of the tube. Different, I know, but just a thought. She needed to be 48" for the ride and she was. She's 7 and still wears size 5 clothes. She's a toothpick.
2...If yes to above...has she ever been on a smaller coaster? Ds is a coaster freak, but was not tall enough at 9. He finally hit the height requirements when he was just over 9 1/2. Prior to that he'd ridden smaller coasters like Scorpion at Busch Gardens Tampa. Heck, he wanted to ride Kraken when he was 5 1/2! Since hitting 56" he's ridden Hulk, DD, Alpengeist, Apollo's Chariot, and most recently Superman "Ride of Steel" at Darien Lake, NY. He is psyched to finally ride Kraken this Feb.
If she's tall enough and "big" enough...and brave enough, I say go for it! Personally, I think my Ds loves coasters because I love coasters and have never instilled fear of them. Heck, at Darien Lake, just 4 days ago, the kid wanted me to ride that "Slingshot" bungee thing with him. Sorry pal, I have to draw the line somewhere!