You could have given him the bill for his immediate expenses and later give him the difference. Or pay him a beer or a lunch to make up for it. I'd never take advantage of a friend in need like that.

And as you said, "it was an easy 15 bucks". You were very aware of what you were doing: easy money off your "friend", instead of friends not minding a few dollars missing here or there.

For people I am good friends with we don't sweat owing each other 15 bucks here or there because it all evens out in the wash.