Hi! I was in this situation a few years ago, and I had to have a "loop diathermy" to remove the cells. It took about 15 minutes under local anasthetic, and the actual procedure didn't hurt at all. I had a period pain type ache for about 24 hours afterwards and took the next day off work to stay in bed and moan about it, although I could actually have gone in if I'd absolutely had to. I had a sort of discharge for about a week - 10 days after, and a heavier than usual period that time, but after that I was back to normal. I had to go for smear every 6 months for 2 years, and now I go for one every 12 months.
The other thing is that you are not supposed to do any strenuous exercise / lift heavy weights for about a week after the procedure, but I had mine done on the Monday, and I was at work putting up solid metal fence panels on the Friday with no ill effects.
Please tell your sister to try not to worry too much, the procedure seems to be pretty effective and :touching wood: I've hqad clear smear tests ever since.