I know a lot of people answered to get a 10' board or something super big like that, but having a board that length is going to be really hard to maneuver. She'll have a super hard time turning around, paddleing out, everything (except catching the waves). She doesn't nee that big of a board. My husband is 6'1" and he was 225lbs (lost some weight since then), and would ride a 6'4" Al Merrick. I don't think you should take your friend out on something like that to learn, it's just an example to show that although weight and height are something to consider, it's not the most important thing.
Make sure you take her to an easy beginner wave on a medium small day. She won't have any fun if you're trying to paddle out in 8 foot faces at HB pier. :)
I'd say go with a 9 foot board instead of 10+. That extra foot will make a difference. Anything 8-9 feet would be comfortable and fun.
I also agree that you shouldn't buy a board. Renting is a great way to get started. Most places charge by the hour, but you can also ask about daily rentals. However, if she really does want to buy one, I'd go with a soft-top or foam board. They're super easy to learn on, they hurt less when you fall, and they won't get pressure dings like a regular fiberglass board will. You can usually buy these used for relatively cheap, too.
Remember, the most important thing when learning to surf, have fun! :)