You as an untrained bystander can do little to nothing.
Professionally trained paramedics could (as an example) pull the leg and thus remove tension between two bone membranes (periostei) and in a best case scenario alleviate all pain. The bone itself has hardly any nerve endings, it is only the periosteum that causes pain. This only works in a few cases. Other options include administration of pain killers.
A few “guidelines” what to do when:
- If the patient is in pain, call an ambulance.
- If the patient can not walk/stand up, call an ambulance.
- If the fracture is open (ruptured skin / blood), call an ambulance.
- If the patient was or still is unconscious, call an ambulance, check for vital functions (breathing, airways).
- If the patient wants an ambulance, call the ambulance.
Bottom line: To be on the safe side, call an ambulance.
What can you do?
During my very first first-help training, my trainer said something that stuck with me: “We humans have two fundamental fears: To die, and to die alone”. While it is sometimes impossible to save someone’s life, we can always, always be with them and comfort them.
So, never leave a patient alone unless there is absolute need, and if so, communicate clearly that you will return very shortly and e.g. just need to grab an AED.
Secondly, being in pain is not very enjoyable, and having a large crowd of bystanders assemble around the patient doesn’t make it a bit better. Try to give them the decency of privacy. If you have already called the ambulance, get a few people to spread out, watch out for the ambulance and guide them to the patient. Hush the other ones away or give them tasks like looking for AEDs in nearby public buildings.
Thirdly, lying immobilised, the patient gets cold quite quickly. Make sure to keep them warm. Jackets, a camp fire, or a rescue blanket are always a good idea, even on mild summer days. (Pro Tip: A rescue blanket makes it quite clear to the ambulance drivers where and who the patient is, and it also protects the patient from the watchful eyes of bystanders. Such blankets are usually a part of every first-aid set in public buildings).
Lastly, as a patient you are very dependent on the people around you. Because patients are usually sitting and leaning against a wall or lying on the ground (to prevent meaningful damage if they should collapse), their field of vision is very limited. Pain also distracts, so they are not highly aware of their surroundings.
This is why one must explain every action one takes to them, especially if it includes coming very near or touching the patient.
TL;DR
Here’s what you could do (order and items on the list depend on every case, don’t take this as a standard rule). Reiterating from above, always tell the patient what you’re doing.
- Check vital functions
- Call ambulance
- Check vital functions again
- Direct someone to get a first-aid set from a nearby public building.
- Send a few people as “beacons” for an ambulance, have them look out for it and guide its way upon arrival at the scene.
- If there’s still a large crowd, send a few people looking for AEDs1
- Ensure warmth of patient.
- Distract the patient until arrival of ambulance.
1: It’s not likely that you will need a defibrillator, and I wouldn’t send for one as a standard procedure. However, it is always easier to send people looking for something than just hushing them away.