Before the series began, she and Charlie used to live together, but then they broke up because Charlie was not willing to be in a committed relationship.
In the episode "Merry Thanksgiving", Lisa tells Charlie that she is getting married. Charlie invites her to his Malibu beach house and pretends to have become a family guy, in the hope of Lisa returning to him. But Lisa sees through his 'pathetic charade'. She does promise to call him if she got divorced.
In season 2 (the episode Yes, Monsignor), Charlie dates a divorced Lisa for a day. He desperately tries to show her that he can take care of her child. However, their relationship doesn't work out again. Charlie says she has "commitment issues" when actually, he's unwilling to commit to Lisa in their relationship.