But isn't the whole point of the 1 to 10 scale that it gives the doctor an idea of how bad the pain is for you personally? I can't see that your friend's method can really work - as you say, pain is subjective to a very large degree and people's tolerances vary hugely. So her pain levels from a dislocated vertebra may well not be the same as mine. But if she classifies it as "8" and I as "10" then the doctor can get an idea of how bearable it is for each of us.
My dentist is always expressing amazement that I refuse injections for nearly all procedures....
Ah, I should've mentioned that this is a doctor that she's seen for years, so he has a history with her pain levels
I think the problem with numbers is that it's so abstract that people will think, "Oh, I don't know...I think this is a 5 level pain..." then a little later, "Oh no, I was wrong, THIS is a 5 level pain, the other one was more a 3....". If a doctor can say, "okay, is this pain worse or better than a bad headache for you?", then there's some basis for comparison. On the other hand, it's hard to compare different types of pain - I was absolutely fine with labour pains, even though they hurt badly during each contraction, because I knew they were each finite, but I absolutely detest bad headaches, because I don't know when they'll end. Not sure about the solution to this.
My dentist actually gives her patients the option of whether to have injections or not (she always asks); on the principle of the above, I've never had injections either, since I'd rather endure the short term sharp pain than have numb, chipmunk cheeks for hours!