Legally, a party which damages another's property through either a deliberate act or negligence (carelessness) is responsible only for the damage that party caused--not for pre-existing damage, or damage caused by other parties. Practically, the issue may be that if the "friend" wants you to pay for other damage and sues you for that money (remember: you don't have to pay anything until and unless you are sued and lose), you will have to provide evidence that you only scratched the paint. To do that, you can use you testimony; your boyfriend's testimony; any experts who examine the truck (though you may need a supoena to get the truck for examination); photographs (if any); etc. (Technically, the friend, as the one suing, would need to prove you did all the other damage; but you'll need to provide at least some evidence and testimony to the contrary to refute whatever evidence and testimony the friend provides.)