Friends - Season 10 Episode 10 The One Where Chandler Gets Caught I'll be there for you.
Overview: Phoebe and Rachel catch Chandler getting into a car with a blonde woman, they follow him to the suburbs. Fearing that Chandler is cheating on Monica, they inform her--and learn that the woman is a real estate agent. The pair has been secretly shopping for houses and struggling with how to tell their best friends. The friends reminisce about their good times in the apartment.