This post is a sequel to: Collaboration or acknowledgment?

The following has come to my attention. A senior mathematician (let us call her Alice) suggested a problem to a young mathematician (Bob) who proceeded to solve it on his own and wrote up the result. Bob agreed to let Alice be listed as a coauthor, but Alice also insisted to include her PhD student (Charlie) as a coauthor because they were thinking about the same problem, despite the fact they (Alice and Charlie) did not even have partial results. Is there a way for Bob to manage such a situation without creating conflict?

Edit 2: Bob had no problem with Alice joining him as a coauthor for the reasons mentioned by Igor Rivin below. Thus, the credit was unfairly diluted by including Charlie who had not contributed. Igor Rivin also mentioned that this is not a new phenomenon, which I am surprised to hear.

Edit 1: One good advice so far is to be generous. It does reward unethical behaviour but I have no better advice for the young mathematician, which is why I asked this question.

Edit 3: I do not agree that it is not useful to discuss these issues in a public forum. If, as a community, mathematicians denounced such unethical practices, they would be less likely to occur.