Author's note: I do not own Numb3rs or its characters. This follows my fics So Long Farewell and You've Got Mail and is set after the first episode of the fourth season.
Probability Theory
There were five sightings in all before they brought it to Don's attention. No one wanted it to be true, but it had happened too many times to ignore it. And no one wanted Don to be blindsided by it in the middle of a case.
Amita had been the first to notice it. She had been running errands near campus when she came across Liz and a blond man having lunch. She had brushed it off, assuming the man was just a friend or colleague of the agent. Then two weeks later, Alan and Millie had seen the pair at a restaurant for an evening meal. Again the incident was dismissed.
It was Charlie who started the suspicions. What he witnessed could have been two close friends or brother and sister, but it didn't look like that to him. Admittedly, he wasn't exactly the most perceptive when it came to relationships, and so he simply filed the information away in the back of his head.
Megan was the one who witnessed the first real proof. She was meeting Larry for lunch when she saw Liz and the blond in a rather heated clinch. The older agent frowned and decided to do some investigating. When she began to talk with the others, things became pretty clear.
But it was what Charlie and Amita saw that really drove them to talk to Don. Charlie had been waiting for Amita to leave the bathroom at the FBI building when he stumbled across Liz and her beau in a rather office inappropriate embrace. Amita had actually walked right into the frozen genius before the two of them made a hasty and undetected retreat.
They convened a meeting of concerned parties at Charlie's. None of them really wanted to broach the subject with Don, but the consensus was that Liz was most likely cheating on him, and he needed to know. Even the math agreed. In the end, it was decided that Megan and Charlie would have to tell Don that Liz was cheating on him.
It went better than they had thought. Apparently, things had been rather strained between them lately, though Don had thought it might work out in the end. Still, it didn't make for a bright, sparkly Don around the office. Between the break up and the whole mess with Colby, they remained mildly concerned about Don. After all, if he wasn't talking to them, who else did he have to talk through things with?