OMG Duperret. That's such a twist that I never thought of that. If it can happen to Grace, it can happen to Rigsby too.
Actually, between all the team members, Rigsby would be my first guess: it's always the innocent one. Or either Grace, since Red John likes to use woman. It might be a really long con using Grace (since she last entered the group) and killing her fiance was also part of it.