A bit late to the topic, but as someone said, it takes more than one healer to wipe a raid.
There are 2 healers, in fact. Unless you saw such WHM intentionally wipe their group.
Either way, you shouldn't have called them out, they weren't on your alliance, you had no idea what was happening on their chat, the party is made of 8 people not 1, you didn't know if they were new and didn't know mechs, etc.
However it doesn't matter much in the end. You made their day even more miserable, but you probably will never see them again.