My dad has had this cactus for over 17 years. It is looking rather sad and it has been a very long time since it has been repotted.

when he repots it, should he separate and keep only the parts that look totally healthy?

He says he never remembers to water it, is that why it is so dried out, or is it some form of rot?

he knows glass keeps too much water for a cactus so will repot in plastic with draining holes. Many thanks!!