Sorry, but little of this makes sense to me.
I assume the tank had two regulators from which the two were breathing. There is no point in having a bottle with pure helium with any regulator on it, let alone two. There is no point in having a serious hypoxic mix, like 10/50, on a tank with two regulators. An hypoxic mix in the 15% or above will not kill you; it will be like breathing air on a mountain. If I input air into a computer at Denver's altitude, I get a warning of a mixture that will not support life, but I breathe it every day with no issues.