I think what your DH did is a good way to acknowledge a famous person in a public place like that, where they are obviously enjoying family time and don't want to be bothered (I mean, Dave was all into it, with the crown on his head and everything!!!). Give them a thumbs up and a smile.

That kind lets them know you know who they are, you like them, but you respect the fact that they are with family and are not going to intrude. I could restrain myself from talking to most famous people, but if I saw anyone who I have been a lifelong fan of or have always admired, it would be tough to keep away!!! I have a male friend who also LOVES Dave Grohl, and he would have probably gone and decorated a crown himself and dressed like a princess just to be in Dave's presence!!