Ugh. It's an intimate moment and a life decision for two people, not a spectator sport. Your friend should ask the girl on a plane or in a stadium if that's what gives them thrills - but not over the PA system. It should be person-to-person. And she should be allowed to accept, think about it, refuse, or whatever she wants without the attention of a crowd of strangers.

I think the public proposal is bad enough in a stadium - but if the partner is embarrassed by the whole thing, at least she can run away from a stadium. On a plane, she's trapped. Double Ugh.