There are a myriad of ways to fake an employment history. By far the simplest would be to state that he worked for a startup which went out of business, hence his need for a job after only 8 months. You could easily just pick a fake name out of a hat (make sure it doesn't exist) or pick the name of one that recently folded which would have zero ways of confirming. It's likely that any company he interviews with won't even quiz him about it.
Heck, he could just say he founded a company that ultimately wasn't able to get off the ground and went out faster than expected...
Small businesses go under all the time so it wouldn't even be a blip on the cursory checks that are performed for the vast majority of jobs.
HOWEVER, there are dragons here.
If this person ever rises to the point of serious responsibility, such as C level. Then the odds increase that the background he puts on a résumé will be examined in quite a bit more detail; not necessarily by HR but rather by share holders. And not just the résumé he sent to a current employer, but the investigation might likely go through prior ones.
Scott Thompson made news in 2012 because he apparently lied about having a particular degree. He did have a college degree, just not the one his résumé indicated. This seems like a minor thing as he had obvious tech credentials from previous employment, however it was the lie itself that resulted in his ouster as the CEO of Yahoo.
So, to answer the questions:
How likely is it to have faked work experience like this found out?
Likelihood is commiserate with the level of job he is seeking and the diligence of HR. Entry/non managerial job - unlikely to uncover if it will take more than an hour. Higher than that? It'll come out eventually.
I think I've only be contacted once about a previous employee. It was a recruiter trying to find someone to say the person was good. Usually HR departments ignore the "who to contact" and simply dial up the company directly asking for their HR department to confirm employment. So unless the HR dept. is also in on this then the likelihood of discovery goes up.
If found out, what would the consequences be to my friend's chances of
Usually immediate termination/dismissal OR an emphatic demand for resignation. If found out during the interview process, then immediate disqualification from consideration. Beyond that, other effects will depend on how often people in your particular industry talk. It would make an easy story to tell so I'm sure it would be passed around.