I will do the same as you and surface with her together.
Never leave anyone alone even at 5m.
How about when it is in conflict with "a thumb's a thumb" or "one victim is better than two"? (serious question)
Although I was not taught to use the rule of thirds in this way, it can be used for the kind of dive you did. If you had said, "I will use one third of the gas I have going out, one third coming back, and reserve the final third for emergencies," it would have worked out okay. You would have turned back at 2000 psi, and had 1000 psi more gas for the ascent. You would have been okay.
I wouldn't have. If I leave 100'+ with 1000psi, I'll be struggling to have 500psi on the boat, even without a panic and air share.