I would hate to take away the surprise here, but he really should find out what she likes before he buys the ring. It is something she will have to look at for the rest of her life and if she thinks its ugly....
I have had friends that this happened to! He bought her a ring he thought she'd like and she couldn't stand it! (they are not together anymore so it doesn't matter...but still)
Jewelry is all about what you want. Hubby and I do NOT have the same taste at all...he is into big and what I think is gaudy...I like simpler, and antique looking pieces. If nothing else they should just go to a mall and hit a store 'just to look' and from that he can get an idea of what she likes.