Marlena, Jada & Rex

This is about Marlena & Jada. Yes, Jada had gotten wind from Xander that Rex never got over Sarah & that she finds out that Rex is Sarah’s ex-husband. But still, how come it was never brought up that Sarah had no clue that Marlena had contacted Rex. So with no clue on Sarah’s part that Marlena had spoken to him, why would Jada think that Rex would lie for Sarah, given that Sarah had no clue he was contacted by Marlena?