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Fall back on training.
"Don't let their problem become your problem."
Is it safe for you to go down there? Quick scene survey- Does the cause of their accident still exist? Are there other hazards that changed because of their accident?
You know you're not going to be able to take care of this on your own. Get help, or send someone in your crew to get help. Have some idea of what to relay to rescue before you contact them - What type of incident, how many involved, extent of injuries (best guess), where you are, a way to contact you, etc.
I've got a phone, radio, and PLB at my disposal. Some others I ride with have a SPOT. 21st century technology, bitches.
Help has been or is being contacted, you've determined the scene's safe, so you approach the victim.
BRING YOUR SHIT WITH YOU! Sounds obvious, but it happens way too often with avalanche rescues... rescuer drops their pack because it's heavy and starts the beacon search, beacon pinpoints the victim, but now your pack is 40 yards down the hill. You need the probe and shovel in your pack. Clock's tickin, bro. Or worse yet, your rescue gear is strapped to your sled and not in your pack.
Use what you have to stabilize the victim, but act within your training and don't "be a hero" only to make the situation worse.
It's a shitty situation to be in, but think things through and don't make any more bad decisions or put yourself or others in a position where you're going to create more casualties.