Yhyjyk

A person I know cheated through most of their science classes while an undergrad. He literally had someone else take the exams for him, write his reports, and do most of his homework. He then took his stellar gpa and not so stellar GRE score and got admitted to a not so great PhD program. His new girlfriend at grad school wrote a lot of his papers. He ingratiated himself with the head of the department - he is a schmooze king. By doing physical work that very few of the other grad students would do he gained favor - cutting open mice heads and placing electrodes in them for whatever experiment his professor needed for his research. His doctoral thesis was weak and the defense round wasn't difficult. He was awarded his PhD. Mind you, he 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...

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

Do I tell his employer? Do I tell the university? Do I let it go? I have no way to prove anything. I'm sure none of those guys that took his tests for him will come forward and tell the truth. He 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.

You should stop beating the horse, because it is dead now. You had a chance but you lost it when you were in school and when something could have done about it. Right now, I believe you don't have any option other than letting it go. I can imagine your frustation in seeing him moving up the ladder (probably higher than you). If you didn't say anything when it actually could have changed anything, you should make peace with it now.

Even if you tell his employer now, what proof do you have to prove your allegation? University probably won't have any interest in pursuing this now. Honestly speaking, I sense jealousy in your post that he is doing good after all this. What you call "schmooze king", is actually a desirable skill in industry. More connections you can make, better are your chances to move up the ladder faster. So I would say, stop being obsessive with his success (with fraud or otherwise), and focus on your career.

You're wrong - he worked hard, just in a different way than you're used to viewing. You're not nearly as good at working a room or a social/corporate/academic hierarchy as he is - he's shown himself to be very good at selling himself, at making sure that people feel comfortable with him. You're basically upset that he's a better salesperson than you are, and you're upset that he got what is, in your words, a job selling things. It sounds like he learned quite a bit while getting his PhD, and is putting those skills to good use.