Has it been ever happened that a university investigated post-PhD problems?

My friend believed that a colleague of her (who finished his PhD) did not mention that she helped me in his work. The thing is that he finished before she finished her PhD. So she mentioned that in her PhD. Now Her colleague is claiming she has no right to mention that since he finished before her and threatened to complain to the university.

What is the worst that can happen?

eh? Too many people in this sentence.
I think the action would depend on whether or not the degree has been awarded. Either way, an allegation can be opened at any time for any PhD thesis, as far as I know.

the degree was awarded for both of them

If either thesis was not that person's own work, then PhDs can be withdrawn after being awarded. See this for an example:
http://www.timeshighereducation.co.uk/story.asp?storycode=400950

Good grief!!!! That's 22 years later! Heck, that had to sting :-/

This is substantial plagiarism..
In my friend's story it is only names of two categories which my firned believed was her ideas before the other guy belonged them to him!