Whether to report someone receiving extraordinary levels of help for PhD 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.)
Getting help is in and of itself not a problem. Proof reading is perfectly fine. Discussing research with others is very much a part of doing research, so having a sparring partner is a good thing. Even finding and reading papers is not a problem if person B is just interested in the research and the discussions. So a lot of the things you mentioned are not necessarily problems. 

If B wrote large portions of the text and A fails to clearly mention that, then that is a problem. But you will need proof before you can take action.