As long as it was just the hood that bounced (and not with the lens attached) I wouldn't worry about it. You can get some cutting compound and then finer finishing polish, but it'll be quite a bit of work and there's no guarantee you'll remove it completely. You also need to be careful you don't make matters worse trying to remove the scratch.
I'd probably leave it alone, explain it to him and if he's really that bothered and you are, you could offer to buy a replacement lens hood ...
Hope you get it sorted out.