Some details are missing in this question. To answer this question the data generated from the intervention would be required. However, even if the data shows improvements in pain following a weight lifting intervention for 2 months there are an unlimited number of confounding factors that may contribute to less pain in 2 months. For instance, is the person taking pain medication? Are other lifestyle factors being considered? Does the person have a strenuous job that may worsen the pain? Is the person actively seeking other therapy like massage ?
It’s difficult to even compare across the subjects because their back pain can differ and can stem from different causes. Some might find that the back pain gets worse with exercise, others not. Lastly, people have different levels of pain tolerance, I may rank my pain level high where as you may rank yours low. Is my pain worse than yours or can you tolerate pain better than me?
All in all, without any additional details your question cannot be answered. Pain ranking is a subjective tool that renders itself not viable. If you can objectively assess pain then we can come to some conclusions.
Not the answer you wanted to hear, but the answer you needed:)