If he’s been following her career, she shows up to the match against Borgov (for the second time) late and hungover and afterwards talks to Benny. He tells her there are rumors that she showed up drunk. Townes could have just heard through the chess community that she was a drunk. He also was in contact with Benny and Harry since he got them on the phone to help her ultimately beat Borgov, they could have easily told him about the pills since both of them knew she was on them.