Surely a negative channelling cleric, blasting their own party, wouldn't be allowed? That is pvp.
As players we have to believe what the GM is telling us is correct, of course there are always areas were players should feel motivated to correct the GM when something seems like a misunderstanding (a cleric channeling negative energy to harm the party and heal her undead minions, GM claiming that a particular enemy is immune to critical hits- especially if this was true in D&D...), but the scenario might always have given enemies specific abilities, that can make things tough.
He means an enemy cleric who damages the party and heals his undead minions AT THE SAME TIME (which he may not do, it's one or the other). Just as an example for "Hey guys, today there was a ruling my GM did I'm unsure about, can someone who played the scenario have a quick look if it was okay?"