After Rachel finds out about Taub's affairs, she finds an online support group and starts exchanging messages with Phil. Taub finds out about it and realizes that Rachel is discussing things with Phil she won't discuss with him. He doesn't so much mind her venting about his affairs, but she's also telling him trivial things about her life (like her family's upcoming birthdays) that she also doesn't discuss with Taub. She makes the excuse that Phil is easier to talk to and that Taub doesn't like to listen to those things anyway.
Things come to a head at Sanford Wells' wedding in Small Sacrifices when Taub accuses Rachel of essentially doing the same thing as he did, cheating, even though she has never met Phil. This finally leads to Taub's decision to ask Rachel for a divorce.
After they break up and Rachel gets pregnant, Rachel looks up Phil and they start a long-distance relationship. After Sophia Taub is born, Rachel and Phil ask Taub to allow them to take Sophia back to Seattle, where Phil lives and works. Taub turns their ultimatum down flat. When they come back talking in a more conciliatory mood, Taub realizes he's being manipulated and turns them down again. He finally visits both of them and tells them that he will not give up his access rights.