A friend told me that she quit drinking after seeing an episode with a plot/subplot of a friend who had a drinking problem. Does anyone remember or know of this episode?

Kevin had a DP in an ep. Belvey of course showed him the way.

A clip from this episode was part of the opening when Kevin's in a bowtie and points sloppily with a big smile.

It was on the 7th Episode of Season 2 entitled "Contract". Kevin gets drunk at a party and upon getting home, his parents make him sign a contract. It's a contract saying that Kevin will not go home by himself when he is drunk and that he should call home so his parents could pick him up. Kevin mistakes this as a permission to drink and he proceeds to drink all the time. But he always calls home according to his contract. Mr. Belvedere sets Kevin straight.

:)

If I was writing the episode, these would some of the lines you'd hear:

(George and Marsha enter Kevin's room with a man accompanying them.)
Marsha Owens: Hi.
Mr. Lynn Belvedere: Ah, Mr. and Mrs. Owens, you're home, and you've brought Kevin's teacher, Mr. Gordon Robinson, with you.
George Owens: Yeah, Wesley tells us you were serving drinks to Kevin?
Mr. Lynn Belvedere: I tried to, but he seems to have lost his taste for it.
George Owens: (surprised) Oh, yeah?
Gordon Robinson: He has?
Kevin Owens: (getting up from his chair) Yeah, Mom, Dad, Mr. Robinson, I'm really sorry. I'm okay now, I guess I was just acting a little more stupid than usual.
Marsha Owens: Kevin, what were you thinking?
Kevin Owens: I don't know. I just thought people liked me more when I was drunk.
George Owens: We sure didn't.
Gordon Robinson: Heck, if those people you were hanging out with really were your friends, Kevin, they wouldn't have liked it either.
Kevin Owens: They wouldn't?
Gordon Robinson: Indeed, not. Kevin, your friends like you for who you are, not what they want you to be.
Kevin Owens: Actually, now that I think about it, neither do I.
Gordon Robinson: (smiling) Well, Kevin, we're all glad that you finally learned that it's always best to just be yourself and that people can like you for who you are.