YES. SHE'S LYING.
Francesca knows that news in the small Iowa town that she lives in travels fast and gossip travels faster. She is well aware that if she admits to anyone that she knows all about Robert and that she spent the afternoon with him earlier...and that he is in her house at the moment, that the gossip would be unforgiving.
Francesca knows how people, especially women are treated in that town if adultery is whispered about. The film shows how Lucy Delaney is treated by the people in town over gossip about her committing adultery with a married man.
Francesca's husband is well respected in that town. She loves her husband and children too much to ruin them in that town with gossip. That's why she acts as if she doesn't really know or care about Robert.