Marty not helping Doc makes no sense
Before Marty travels back to 1955 (instead of Doc deciding to not invent the time machine anymore, after all, he's already seen so much of it, and how it just causes so much trouble, danger and pain), he doesn't even consider slipping Doc a small note listing all the rich corporations, companies and stock that will be valuable in the 1980s.
I mean, Doc doesn't have to use that list, but he could fund himself and all his inventions SO easily by just investing in the correct stock at the correct time, it's mind-boggling that Marty doesn't help him save his mansion and family fortune!
Just a tiny note with a few lousy names, and he won't write it?
What's wrong with Marty? Does he not care about Doc or the future? I know he was 'slightly lectured' earlier in the story about monetary greed, but surely that's not enough to erase the idea from his mind completely, plus, this would be for a good cause, and not harm anyone.
Plus, as I said, Doc wouldn't have to use it - Marty could just say, here's a list of big, rich, wealthy companies in 1980s, so if you cheaply buy lots of stock of these companies, you can sell them for a lot of money and not have to lose the family fortune or your mansion.
Of course, if Doc decides to never even invent the time machine, he could be just a super wealthy inventor, living a very comfortable and happy life without all the problems. Heck, he could afford to just BUY the plutonium!
All these movies break the whole idea that you shouldn't know too much about the future (but a little bit would be fine?), so why not?
Also, as I mentioned, Doc wouldn't have to use it or even look at it. He could take the list, put it in some drawer, and have it as an 'emergency plan' if something goes wrong, that he could then harmlessly use. It could be left to be Doc's decision, also simultaneously testing Doc's character and integrity - does he only speak, or does he practice what he speaks?
Marty is a really cold-hearted !@%#¤* for not helping Doc this way, and thus their whole 'friendship' makes no sense.