This post is a sequel to: Collaboration or acknowledgment?

The following has come to my attention. A senior mathematician suggested a problem to a young mathematician who proceeded to solve it on his own. The senior mathematician signed the paper and included his PhD student as a coauthor because they were thinking about the same problem, despite the fact they did not even have partial results. Is there a way for the young mathematician to manage such a situation without creating conflict?

Edit: 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.