The woman being groped is Ann, his wife.
George Smiley's relationship with his wife is famously complex / bizarre over the books they appear in.
In the movie he is currently estranged from Ann. They are not living together. He is aware that Bill Haydon had an affair with her in the recent past and that memories of this appear in the movie.
Another important scene relating this is where George remembers discovering Bill at his house, clearly getting dressed and looking embarrassed, surreptitiously forcing his feet into his shoes. He makes a weak excuse about returning a painting to her. In the books we are told that he is a cousin of Ann, so has some legitimate reason to know her. The painting is the one that we see Smiley spend time looking at.