In the US, you don't have to do anything. If they are not responsive without a pulse you can legally administer CPR with the "good Samaritan" laws. However it's extremely unlikely to actually work in the best of conditions. We were taught as EMTs that it's really for the bystanders to have something to do. Unless there's an AED, it almost never works, but hey you may as well try.
I am a volunteer EMT, so it's worth it for me. But I also understand it's very likely that even if you return the patient to spontaneous circulation, they might still not live or might have serious issues like brain damage. Cardiac arrest is often caused by something else.
Also it's not just sore arms, CPR on an elderly patient will often crack ribs which is pretty upsetting. A 9/10 chance you'll feel like you killed a person by crushing their chest is pretty high.
Mouth to mouth is really gross and I wouldn't do it to someone with vomit all over them. Without a suction device it's not effective anyway.
I do think everyone should have to take CPR+AED in high school and college, it's a great skill to have and shares the burden of basic life support around the community. But I totally understand why someone wouldn't want to perform it.
