Mary and John broke it off after the death of their daughter. They still remained close, but their friendship was always slightly strained. And then, after awhile, they just fell out of touch with each other.

Then, all of a sudden, they got back in touch post-TRF. Their friendship resumed and they were due out to dinner when, out of the blue, John stands her up. Concerned for her ex-husband, she goes to 221B Baker Street and finds, much to her surprise...

John and Sherlock in bed together, in a very compromising position.

Cue potential fall-out.