Nope, nothing specific like that. There are the Aid Another rules, which can let you give someone else a bonus to their own Athletics or Acrobatics check to escape, but that wouldn't help someone who's unconscious.
If you have a power that would push/pull/slide either the monster or the PC, that could break the grab if the PC ends up outside of the monster's reach.
You could use the rules for moving a grabbed target, I suppose. Make a grab attack against your ally (and if they're unconscious or willing, I'd house rule that your grab automatically succeeds). With an unwilling target, you would then make a Strength versus Fortitude check to move at half speed with the grabbed target, but in this case I'd say you could make either an opposed Strength check against the cube or Strength versus the cube's Fortitude to wrest the dying ally free. If you're feeling generous, you can allow Athletics instead of pure Strength (which helps if the PC is trained or gets some other Athletics bonus).
Personally, I kind of like the opposed Strength checks idea - playing tug of war with the cube's toy. But that's my own house rule.
I think the way you handled it is totally reasonable, too.