you should definintely mention it to your dad - the friend he bought it off may have said it had never been used or something, in which case your dad may have been ripped off. also, the friend may have the warranty which will mean it can be fixed. I know you don't want to upset your dad by saying its broken, but maybe just give it a mention (make a joke and say you've probably used it too much since he gave it to you or something?). COuld also try the other idea of popping into the shop wtih it to see if they can fix it first? good luck (and happy birthday for yesterday!) xx