I had the same initial reaction but I think the doctor is treating it as a rating and not a fraction. E.g. a 90% grade doctor would recommend it, but they're a 20% doctor.
It's a different joke. He's saying a doctor with good ratings like 9/10 would say to stop, but he has bad ratings, 2/10, so he's going to give bad advice.
I have never heard of "2/10", but after a bit of research, I see a couple very loose connections:
An accounting term regarding a discount for prompt payment. I do not see reference of this being used in a drug context
Smoking something and rocking the pipe back and forth between the "2" and "10" positions on a clock. I do not think this applies to cocaine, or even "crack"