CoQ10 is an enzymatic cofactor and also a protective antioxidant. CoQ10 has been linked to, or has been hypothesized to, protect against heart disease, cancer, and Alzheimer's (among other less headline-grabbing things
The reason statins make you deficient in CoQ10 is because (in total layman's terms) when the statin cleaves the biochemical pathway that makes cholesterol (in effect turning off your body's ability to make cholesterol), the part of the pathway that is responsible for the production of CoQ10 is also below
--or "downstream" from where the cleaving occurs. So a side effect of cleaving this pathway is to not only shut off cholesterol production, but to shut off CoQ10 production as well. Theoretically once this pathway is not being interfered with any longer, CoQ10 production should resume.
Whether he still "needs" the CoQ10 is unclear to me, though--I don't know that there's a way to test for how much you are producing and/or what optimal levels are. I have naturally very low cholesterol--my body just produces very little--so using that as a kind of "proxy" for my CoQ10 production, I worry that my levels are too low, even in the absence of a statin.
One reason folks tend not to supplement with CoQ10 is the relatively high cost. IHMO, if cost isn't an issue, I would keep it going just to be on the safe side.