The only way I can think of him knowing anything about her is through Claire. Other than that, there's not many reasons for anyone to know much about Ada. She'd be a lousy spy if she was a household name.
Ada was never one of Wesker's top agents. She worked with him like once, in RE2.
Ada worked for Wesker in RE4 too iirc. its why she was peeved that Wesker sent Krauser to act as her backup.
in any event, the fact that Chris didn't know about Ada makes me very suspect of Leon. Chris knew about all sorts of things in RE5 from what he himself called as "the Kennedy reports", which i would believe would be a briefing detailing the events of RE4. for Leon to omit Ada Wong in said report speaks of how he is fairly compromised as an agent since he is willing to omit certain information that other branches of security might find useful due to an emotional attachment to said information.