How is he going to line it, and with what material? What is the line made from? What size is it now and what size will it be when it is lined? How is he going to get around the corner? What warranty on the lined pipe and who is responsible for fixing it when the lining fails? Will he do a post-installation video inspection, and give you a video tape for you to keep, showing that it was done and effective?
It is very hard to clean a line in place so you will get good adhesion of the liner. Some lining systems pull a completely new line through the old, even fracturing the old with a device pulled by a cable. That usually doesn't work going around a 90 degree bend.
If the lining is installed by jetting out the old and putting some kind of pookey inside that is supposed to stick to the old, then it probably won't last long.
It is not something that your local plumber is likely to have equipment to do. Take a look at how it is done is large scale work (see links). Then ask how he will do it and what it will cost. Get an estimate for replacing the line and compare the prices.