Billie ends up dating Brian for a short while during "The Third Man" episode, but she dumps him when she finds out he's still involved with his ex-wife.
Billie: What are you doing? Zack: I'm looking out for you. Billie: Well, stop it. I have been dating since... oh my God! Since you were in kindergarten.
(to Zack) When I saw that guy kissing your pregnant lady, I got this really sick feeling in my stomach. Kinda like the time I saw my mom skinny-dipping with my Uncle Bob.