What to do with someone that cheated their way through university and a PhD program? A person I know cheated through most of their science classes while an undergrad.  They literally had someone else take the exams, write reports, and do most of the homework.  They then took their stellar GPA and not so stellar GRE score and got admitted to a not so great PhD program.

A new significant other at grad school wrote a lot of their papers.  This person ingratiated themselves with the head of the department – they are a schmooze king.  They gained favor by doing physical work that very few of the other grad students would do. Their doctoral thesis was weak and the defense round wasn't difficult.  They were awarded their PhD.  Mind you, they still can't explain to me nearly anything about basic chemistry,  organic chemistry or biochemistry concepts.  I'm talking like sn1, sn2, benzene rings, buffers, etc...

This person now has a job with a pharmaceutical company doing anything but real science.  They're basically a pharmaceutical rep out selling drugs to various physicians in a territory. 

Do I tell their employer?  Do I tell the university?  Do I let it go?  I have no way to prove anything.  I'm sure none of those people that took the tests in their place will come forward and tell the truth.  This has gotten away with fraud.
He literally had someone else take the exams for him


If you wanted to speak out, that was really the right time, not so many years later when damage is done and every accusation is virtually unprovable.


  His doctoral thesis was weak and the defense round wasn't difficult.


There are many weak PhD theses around, and it's up to the universities and to the defense committees to decide which level of weakness is acceptable before rejecting a candidate (gosh, if we were to object to any "weak" degree...).


  Do I tell his employer? Do I tell the university? Do I let it go?


You let it go because, as you said, you've nothing to prove your accusations (and the university would have no interest in undertaking an investigation after such a long time without any evidence to start with). And he is now in industry, and it's up to his employer to decide whether to promote or demote him according to his work performance and to decide whether the PhD title is just a vacuous document in his hands.

Different is the case where, for instance, you can prove that a student cheated by plagiarising other works. Here, there is evidence that can be used to revoke a degree, as it happened in some cases.