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. You need to build some cred. One way is by having a Fagrorate relative to other pros. Another way is to enter a pro event and beat a player considered a "pro". Getting sponsored is yet another. It's not as simple in pool as signing a contract with a "pro" team like baseball, or even paying a membership fee to an organization, like in golf. Alternatively, one could register at AZ Billiards and become a Gold Member. Then you can write off the expense.
Something else to muddy the waters. When it comes to sports, our definition of "pro" can tend to be a little skewed. Mainly due to how "pro" teams market themselves. Amateur baseball players, or minor leaguers, for example, are still pro baseball players.
I'd say, if "your buddy" wants to call himself a pro, let him. There's unsuccessful "pros" across the board.