The distinction of "pro" is not a black & white proposition. Technically, anyone choosing to pursue a career playing pool can consider themselves a pro, regardless of skill. However, that doesn't mean others will. Another way is by having a Fagrorate relative to other players considered pros. Or, by entering a pro event and beating a player considered a "pro". Getting sponsored is yet another. In pool it's not as simple as signing a contract with a "pro" team, like baseball, or even paying a membership to an organization, like in golf.
I'd say, if your buddy wants to call himself a pro, let him. There's unsuccessful "pros" across the board.