Glad you both made it okay - this was potentially a very bad scenario.
First off, what is the water temperature in the lake? In very low water temperatures, freeflows are common. You had a strategy for a freeflow -- you had a pony. But your buddy, although carrying far more gas, had no strategy, because he could not reach his valves, and he did not tutor you in how to assist him before you dove together.
Second, this is a lake, right? Why are you doing a hand-over-hand descent down the line, which requires that you and your buddy be at different depths, and therefore makes it far more difficult for each of you to see the other or understand what is going on with him? If you had done a descent facing one another, at the same depth, the freeflow would likely have been immediately obvious to Ian.
Third, Jax makes a very good point about communication in the dark. No one can see hand signals well unless the hand is illuminated. (This is another reason for being able to do the descent without hanging onto anything.) But you did have a light, and a rapid, urgent light signal, persisting, should be a good indication to any buddy that something is very wrong -- which is really what you needed to convey to Ian.
I believe the standard Aluminum 80 is a 12 liter tank, right? So a 3l should have been about 20 cubic feet of gas. At 15 meters, that should have given you enough gas to get to the surface easily, even if you wanted to make a stop. Did you consider switching to the pony immediately, and getting Ian to turn off the main tank? You probably would have saved significant gas, reduced your stress level, and after a minute or so, if this was temperature-related, the first stage may well have thawed enough to use again. But you'd have been on the way to the surface on your pony, anyway.
The point about being familiar with one another's gear, and somebody being able to understand a manifold and do shutdowns if necessary, has already been made.
Again, glad you both came back safe.
