So I think Booker's sympathies don't develop until this plot starts, if that helps. He's a very ambitious social climber who's keen to become a cardinal and at first, he was just using margo for her connections then... FEELS. here's his background for more info for stephen to know.

so i'm down with stephen knowing about her relationship with booker, she probably wouldn't hide it from stephen anyways. but she'd also be keen to keep booker away from him once she realizes how booker is feeling and how she might be feeling because of booker.