My diabetic mother has excessive pain in a root canal treated tooth. Is this normal?

Hello doctor,

My mother is 64 years old and diabetic for 12 years. Recently she visited a dentist to get a denture. The dentist told that two pre-molars (1 up and 1 down on left side) have to be root canalled even though there was no pain in them because those teeth were not in good condition which he said after looking at an x-ray. I do not have any of those x-rays. So, the root canal was done on those two teeth, and cotton was placed there and asked to wait for one week. There is no pain at all. After one week, the cotton was removed, and the cement was placed. Within a few hours, the pain started, and it was increasing pain. The dentist then removed the cement and again placed cotton and asked her to wait for one week. No pain at all. Again on cementing, within few hours pain started a lot. Again cement was removed, and cotton was placed. This cement and cotton alternation has been going on three times (for three weeks), and the dentist says it will work at some point in time, and the cement will be permanent. He says the pain during cement is due to the gas pressure from the gas release by the microbes sitting on the remaining disintegrated pulp. If even finally it does not work and pain does not go away while with cement, he said he would make an antibiotic injection or spray on those teeth. I do not have any x-rays, so this is the only information I can give.

My question is, Does this procedure look normal and logical?