Lots of jokes come to mind....
... its just a flesh wound - suck it up.
... oh man, that's gotta hurt.
... man, you are so screwed.
Remember the #1 rule of rescue - don't make a second victim. I'd say layer up the rescuers but my own adeneline in these situations could keep a dead horse moving for hours. The real risk to me comes later.
Get the primary survey done in 20 seconds.
Get them off the ground sitting on a pad or their pack and into their emergency gear in less than 5 minutes. Use questions like, if I help you, can you sit on your pack? The real risk here is the environment not the broken leg.
Complete the secondary survey and determine the course of action then call or send for help if you have the resources.
Stats will show that your (his/her) chances of survival are not too good above treeline. Therefore, as mentioned by others, the priority is to stabilize the injury and get them below treeline quickly. I recall the prayer from "The Patriot" "Lord make me fast and make me accurate." (?) Hypothermia will creep up so quickly. If the shock spiral starts your screwed. It might be better to walk them out in pain. Pain and fear are great motivators especially when served with doses of confident reassurance.
If I can help them down with an arm over the shoulder, not loading the foot I would try this right off. Might even do it without splinting the leg but the "official" position is to splint first. I don't think I could make BUF splint it in the time I have in those conditions. Of course a roll of duct tape and a separated pole might do the trick in the time available though I usually carry neither.
That little steep part below the summit is the crux. Getting a leg injury down that would be tough. Too narrow to walk side by side. I'd like to think I'd improvise with my twin 5mm lines
but in reality, I think I would do series of hug steps where they hop on their good leg. I'd go down first just in fron of them facing the slope and together lower them down to me. Definitely put on my crampons first. There is nothing that gives you the confidence that you get from stamping a 12 pointer into the ground.
Below treeline. Eat and drink and pee. Generally, broken ankle is a self rescue. Here I would beef up the splint and prepare for the walk/drag out. I would favor a travois drag set-up hooked up to my pack. With enough food and water I could pull that for days.
I had a similar training scenario in WFA and everyone died.
The times I had rescues they all lived, but I owe that to the learning from WFA. I wrote an article about a winter emergency self rescue we did once and could post it if there is interest.