there is a grey area for coins, of he attempted to pay for the bill with it then yes. since he left it as a tip, no. you could probably get a court case going but it's legal to modify coins as long as you don't try to commit fraud with them. if you took a 1 dollar bill and wrote two 0s to make it 100 then yes. if you signed it or marked it with a marker then no. it's all about fraud and intentions.  tldr, no one cares about pennies about coins along as you are not committing fraud.
Bulgaria