As several people have said, there really is no practical limit to the CESA in terms of depth.
On the other hand, a joint PADI/DAN study a few years ago found that the #1 skill related cause of death in scuba was embolism following a rapid ascent, usually after an OOA episode. It is not the CESA that was at fault, though--it is the failure to do a CESA properly that caused it. If you hold your breath when you do it rather than exhale the whole way as you are taught, you are at great risk.
You mentioned that they did their ascent with their regulators out of their mouths. That will work just fine as long as they continue to exhale all the way, but it is not a good idea. If the ascending diver blows out all the air too quickly and succumbs to an overwhelming urge to inhale, the diver with the regulator out of the mouth will inhale water and drown. The diver with the regulator in the mouth will probably inhale air, because the OOA regulator was not actually OOA--it just could not deliver its little remaining air at the greater depth where the incident began. In the worst case scenario, the inhaling diver will get nothing, which sure beats breathing water.
On the other hand, a joint PADI/DAN study a few years ago found that the #1 skill related cause of death in scuba was embolism following a rapid ascent, usually after an OOA episode. It is not the CESA that was at fault, though--it is the failure to do a CESA properly that caused it. If you hold your breath when you do it rather than exhale the whole way as you are taught, you are at great risk.
You mentioned that they did their ascent with their regulators out of their mouths. That will work just fine as long as they continue to exhale all the way, but it is not a good idea. If the ascending diver blows out all the air too quickly and succumbs to an overwhelming urge to inhale, the diver with the regulator out of the mouth will inhale water and drown. The diver with the regulator in the mouth will probably inhale air, because the OOA regulator was not actually OOA--it just could not deliver its little remaining air at the greater depth where the incident began. In the worst case scenario, the inhaling diver will get nothing, which sure beats breathing water.