This is a real life scenario, but I'll refer to the people involved as A and B.
A and B are both retired. A is doing a part-time PhD (humanities), but for at least a year* B has been working nearly 9-5 every day helping A. The help seems to involve research (finding and reading papers/sources), discussion and input, proof-reading and re-touching the thesis, but I don't know about actually writing large chunks although I wouldn't be surprised.
As far as I know Masters and PhD theses usually need a declaration saying something like "this work is all my own except where explicitly marked". To me it seems as if the declaration would be false, and anyway, even if person A declared they'd received significant help presumably joint PhDs aren't granted, and B isn't enrolled on a course of any kind.
What's my best course of action? Ethically, I feel as if the university should know about this, although presumably the fact that a lot of work has been done (by whomever) shouldn't go to waste.
(*The help has been going on for years, but I only know the specifics of the time invested for the past year or so.)