Did the fentanyl stop his breathing, or did the meth stop his heart?
Remember dose makes the poison. If the drugs in his system had been sufficient to do either of those things, it would have said so with cause of death as overdose.
And that would still not exonerate an officer who kept his knee on the victims neck for nearly three minutes after he became unresponsive/no pulse instead of rendering 1st aid.
How long can you hold your breath?