a silmilar thing happened to us (well almost). we were at 80-90 feet by the cathedral rock and this other diver (a korean and not my buddy) showed me his gauge. it was under 700psi and to think that we just got there. our instructor asked me to escort this diver back to the surface sharing my air. it turned out that this diver forgot to change his tank after the 1st dive... well it was not scary for me...more like pissed because he ruined my dive...hahaha