I think public proposals are bogus and don't mean much.
I agree with this, but there is an exception. If they have already been talking about getting married beforehand, then I think it is all fine and dandy, especially if you know your girl likes to be the center of attention.
Due to the length of their relationship, it doesnt seem likely that they would have discussed it already, and you can also tell by the reactions of everyone around, they were all shocked, including her. I hope it wasnt a complete surprise to her and hopefully she knew he was going to propose at some point.