Your friend has probably not "seroreverted" (gone from being HIV-positive to HIV-negative.) More likely, his doctor has told him that he has undetectable virus. The two are very different, but unfortunately many people mistake one for the other.
To "serorevert" he would have to have lost his antibodies to HIV. This would mean that both the ELISA and Western Blot would have gone from positive to negative. This does not happen in adults unless they are just so sick from AIDS that they no longer are able to make any antibodies at all, and that's definitely not a good thing. Of course, we don't continue to perform ELISA and Western Blot tests in an individual who has well-documented HIV infection-- one more reason why it's unlikely that your friend is talking about loosing his antibodies.
It is not uncommon, however, to have undetectable virus in the blood. While this is a very good thing, IT IS NOT A CURE! It simply means that the amount of HIV virus in the blood is so low that it can't be detected with the standard assays, which can measure virus in concentrations as low as 100 to 10,000 copies of virus per ml, depending on which test you use. Having undetectable virus could be either a result of a healthy immune response or to effective antiretroviral therapy. If you were to do more sensitive tests, such as viral culture, non-quantitative PCR, or lymph node biopsy, you would still find evidence that the virus is there, but in low levels.
If someone is on antiretrovirals and has undetectable virus, chances are that there's a connection. It means that the drugs are working well to suppress viral replication, and stopping them would not be a good idea.