Unless the reason for the firing is a felonious offense against the previous employer, then let the sleeping dog lie.
Getting canned for telling off his supervisor is one thing, but getting caught embezzling or being involved in bribery is another, and reason for concern.
How about:
Resume contains a lie?
Been that way for years?
Stellar employee? - Who cares. See above 2 lines.
Given that they changed from Robin & Anne to Robert & Andrew in a very short period of time…
I fail to see how that is relevant at all. Either the HR manager didn’t check for some reason, or didn’t feel it necessary to do so in making their decision. I also fail to see how everyone doesn’t have plausible deniability in this instance. How is it going to “come out” that the boss knows? Is Mickey under investigation for something? What company is going to randomly re-investigate an employee’s background without cause?
But then you wouldn’t have attracted all these wonderful responses from horny dyke-porn fetishists. I don’t know how many gay-porn fetishists Robert & Andrew would have attracted, though, so you may have a point. Robin & Anne may be negatively impacting the thread. There’s an idea for your next poll thread… Don’t thank me, it’s the least I could do.
If I’m Robin, I voted “kick it upstairs.”
But before I do that, I’d call Imagine (or whatever the other company is), tell them that I’m checking Mickey’s references, and ask them to confirm the date of employment. If the dates match his resume, I’d assume Anne was mistaken and let it go. After all, her truthfulness is not exactly stellar either. But if Anne’s correct, then Mickey’s got a date with the VP in his future.
If I have knowledge, I have to act on it. It’s my job. It’s not my job to decide if Mickey stays or goes. I’m not going to put my job on the line for him. I’m not going to cover up for someone who’s already lied to me - no one ever just tells one lie. If I cover him, then both our jobs are suddenly on the line, and inevitably, shoes will fall.
However, I would tell him in person what I had found out and the actions which I was instigating. If he had a good story, I might be willing to go to the VP with him and add a plea for clemency. I wouldn’t just rat him out behind his back.
And then I’d dump Anne for outing Mickey even though she’s desperate to avoid being outed herself. Tattletales suck.
Yeah, see, that’s a big lie. I’ve heard of other cases with fictional graduate degrees from schools the employee never attended.
I don’t see Anne as having deliberately outted Mickey. She doesn’t work for Nunya; how was she to know he lied (or had a typo) on his résumé?
He could have lie about actually having worked, gone to college, been an astronaut, etc*… If he’s as good as the description I would quit before I allowed him to be fired. Resumes are handy for weeding through massive piles of applicants. Once the job starts, all that matters is if they can or can’t hack it.
*For most things. If he lied about going to med school and is now a doctor, I’d be slightly less happy. 
(bolding mine)
What possible good would your quitting do? I mean, you have the option of appealing to the VP and arguing Mickey’s case; why have you any further obligation? It’s not like he lost an arm saving your life during the war.
This particular scenario is a no-brainer to me: pretending not to know about the lie is the only reasonable choice. That wouldn’t always be the case, but factoring in the excellent performance, the magnitude of the lie (manipulation of dates versus blatant making up of a job or credential) and messy consequences of attempting to explain how you got this information means that doing nothing would be the best course.
If the lie was more elaborate I wouldn’t be so quick to answer. If they had made up a university degree, complete with forged paperwork, I’d be more inclined to fire them. An elaborate fraud attempt is reasonable cause for concern about what they might do with increased responsibility, and that lack of trust could be a deal breaker. It may even cause me to question their perfect performance.