Unfortunately, a lot of people pick DPS classes because they feel it's low responsibility, that they can slack off and nobody will even notice, let alone call them out on it. The trick is to treat DPS exactly the way they want to be treated, which is as useless, replaceable peons that nobody would notice if they were missing. It's a low responsibility tool that's easily replaceable, so don't worry about them getting offended, and if they continue to mouth off then just get a new one.