I can't remember if it was Lisa or not, I don't think it was though. Probably just a one-off-episode girlfriend. Anyway, If she got a one-off part it probably wouldn't have affected their relationship much, apart from Will would have known Jada better, remember, at this time he was still in love with and married to Sheree (I think).
If it was for the long-recurring part of Lisa it would definitely have changed their relationship. I think it would have been harder for them to get together in real life, becuase they would have soooo much media attention, for meeting as "boyfriend-girlfriend" in a sitcom, who knows how it would have worked out?!