If they work for a French soccer team, that just makes them cross-border commuters, which typically means you're responsible for taxes in
both countries (unless a tax treaty between the countries harmonizes them). A Monaco resident working in France still has to pay French income tax on his or her French salary & bonuses. Living in Monaco may be a way to dodge taxes on advertising income if they do endorsement deals, though.
Also things are more complicated for French citizens in particular, because Monaco and France have a tax treaty (dating to the 1960s) aimed at making it harder for French citizens and companies to use Monaco as a tax haven. Essentially Monaco agrees to tax French people/entities at the current French tax rates, rather than the usual Monaco rates, in a range of situations that fit the heuristic of using Monaco as an address of convenience (vs. bona-fide living / doing business there).