A Triangular bandage (40" x40" x56" is standard) can be used for many things. Mainly as an arm sling. A bandana (mine 22 x 22) isn't large enough for a sling. Do you carry a triangular bandage in your first aid kit? I do not, but am thinking about it. I used to be an EMT and (rightly or wrongly) I am expected to treat others when in a group.

I don't. When I took a wilderness first aid course this was something we practiced improvising. If you are on a multiday trip, you usually have at least some extra cloth (clothing, jacket, raingear, ground cover,...) I would rather make do in case of an emergency than carry something large like that all the time.

I haven't had instruction on how to properly sling an arm, so I only carry the most basic things in my FAK like bandaids, pain pills, moleskin, etc.

I do think it's interesting that being a former EMT, you are expected to administer first aid. Is there any legal liability if you do or don't? Or is it just that your friends know you do it better?

I'll admit that I called over a friend/former paramedic when a guy in the office was having a seizure. He jumped in and did some basic triage and scattered the dozen or so well-meaning officemates who only stood nearby unable to help. The paramedics who were called arrived a couple of minutes later and he transferred to them. Is that the expectation?

I only called over my friend because I was worried someone was going to just try to stand the guy up like always happens in TV and movies.

I think BZH has it right. We might be able to improvise...but to be honest, I think an arm injury is the least likely to be an issue in the backcountry. Our FAK is composed of a few things that we absolutely know how and when to use...

I carry a fairly large lightweight camp towel that I think would improvise quite well as a sling. It's big enough to wrap around my waist to cover up temporarily.

If you are currently a licensed EMT or EMT-P (Parametric) in my state, Washington, then you have an obligation to stop at accidents and lend aid to others, even off-duty. If you are not currently maintaining your license, like me, then you have a choice to lend aid or not. If you do start aid, however, then you can not leave the patient until you pass she/he off to a higher medical authority. You can not ABANDON the patient. That might create a dilemma if you were out in the woods and wanted to go for help and no one else was here. (personally, I would never leave the patient alone, never ever.)

If you voluntarily help a person in Washington State you are protected by the "Good Samaritan" law and cannot be held liable unless you do something ridiculous like put a tourniquet around their neck and kill them on purpose.
BTW... you did the right thing by getting someone with medical training. When you have medical training (even first aid training) you at least know what to do and that is the key to keeping calm and taking control of the situation.

I agree with you that an arm injury requiring a sling is not likely unless the person already has a bad joint that comes dislocated. And improvising with the resources on hand, I think, is always the key to self-reliance and a necessary skill traveling in the wilderness; especially for the ultra-light people.

Sounds good PerryMK. Multi-purpose equipment is key to keeping things light and still having what you need. I guess I have thought a couple of bandanas would do the trick. I only carry one, but my partner does too.