If she admits to it, Lisa lose credibility. If a player figures it out somehow (without Lisa admitting to it), they lose credibility. Because of the secret being instant canon, I would assume you guys would want to make the statement something big. Something big is always hard to believe hence the lose of credibility. But no worries, one can gain the credibility back. The whole point of the secret is to be an A-Bomb.

P.S. In the case of a player figuring it out somehow and Lisa admitting to it afterwards, both of them lose credibility.

EDIT: Yeah, somewhat few yeas back :P