If your pet was tanking and it didn't get healed, then the priest screwed up. If your pet was DPSing, and decided to sit in an AOE, and act like a stupid AI script with no human controller, I don't give a damn, and will happily watch the stupid thing die.
Basically, if your pet is not tanking, and it dies, don't ***** to the healer, they probably knew and didn't care. If and only if, the pet is tanking AND there's a wipe where the pet never got any heals, there's a problem with the healer.