Two most likely conditions:
1)Alt, P/S pulley wobbling causing false misfire signals--they need to remove the engine belt and see if the misfires persist(make sure they don't run it too long)
2)Burnt exhaust valve seat. Have them do a cylinder leak down test. This will mean a cylinder head replacement(they're on backorder from GM).
Obviously, I don't have psychic powers and these both could be way off base. Two fairly common diagnostic tests listed above will at least get them started.
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