Most people would likely say they'll do the heroic thing and try to save their friend despite the risk to themselves, hell even I like to think that I would try. But when it comes down to it, it's basic human instinct to save yourself, as fear sets in and you panic as to what will save your life. Most people who say they'd save their friend would cut the rope if the odds where so bad, this includes myself who despite liking to think otherwise realistically has to speak true to the odds.
It's kinda like saying if there was a zombie Apocalypse theory going around the room and people talk about what'd they's do, and people say they'd go gung hoe and charge the horde to save their friend them or give up and let them eat them, no, that wouldn't happen. First time you see a zombie your going to piss yourself and run like hell, unless you have a gun in which case you'll shoot it and run like hell. Even later on you'll never allow it to eat you alive, you'll do yourself in rather than suffer the pain of being cannibalized.
Again the whole point is, not matter what we say, no matter what we think, unless we've seen in these situations or have been trained otherwise (and even then nothing is guaranteed) you're more likely to end up saving yourself than your friend as the panic sets in.
If you want to be heroic, it'd be much easier to be the person on the other end, injured on top, conscious fellow on bottom. In which case you can cut the rope killing yourself and give you friend the chance at life. Granted you'd probably regret it every terrifying second until you hit the ground, but again it'd be easier in that situation to be the hero.