"...but people who have called him their friend, and have benefited from his friendship, have a moral obligation to stand by him..." I agree with most of what you have said, but I disagree with this. I think the decision to stand by someone is highly dependent on circumstances, though if I were his friend and I didn't know about this stuff (very dubious, seems like everyone knew) I would stand by him until there was proof, and at the least, wouldn't throw him under the bus.
There are different ways of standing by a friend. Jesus said we should visit people in prison, so I don't think deserting friends is ever an option. I am skeptical about the “rape” charges and would be inclined to believe Harvey Weinstein that he at least believed his sexual contact was consensual. However, what has been described was immoral, boorish, adulterous, unethical. If I were his friend, I would tell him to take his lumps. I wouldn't denounce him publicly, which I find to be grandstanding. I would NOT have encouraged him to try to save his job. He has years of work ahead of him in the form of repairing his relationships with his children and his brother. he should try to save his marriage. Also, he is Jewish and probably has given generously to Jewish causes. It's time to start practicing Judaism. Of course, he should make financial and personal amends to any women he did embarrass or harass.
I know people who know Harvey Weinstein. He has been personally helpful and generous to many people — and I am not snarkily alluding to sexual tit-for-tat. Those people owe him loyalty, even - perhaps, especially — at a time like this.