A friend just became unconscious because of hunger. I gave him some food and drink, then tried to have the defib work on him. I couldn't turn it on even though I had a worn battery. He managed to walk albeit he was unconscious! I took his stuff, and told him to disconnect from the server, but he reappeared on the shore.
A few time ago I had the same problem on a dead player, I managed to turn the defib on but couldn't revive my friend.