If you're worried about ruining the friendship, then you only really have two choices: do nothing and be angry, or find some clever way of letting the police know about it without giving it away that you knew beforehand (eg you can wait until the event has started, if it's going to be visible anyway there may be a few others noticing anyway; or let somebody else know and "suggest" that they might wish to tell the police, so that you can honestly say that you didn't tell the police about it).