I could see Dana being a friend of Lisa's but having a crush on him.

I seem to recall dana saying something along the lines of "Lisa's my friend too and she can't stand me." so I'm sure they all met in college or there abouts. Quite a few of the stories I've read have had Lisa and Dana as room mates, which would seem right.

She wouldn't be the type of girl to try to break them up, but if they broke up, and Casey started being charming to her (as we know he can), I can see that happening.

Also, in Dear Louise, we learn that Dana went to series of exclusive girls schools so she might have been rather inexperienced around boys who weren't her brothers when she got to college. Imagine shy little Dana Whitaker, fresh from an an all-girls academy and used to men who treat her like a brother, adjusting to a co-ed dorm and suddenly being confronted with the slightly dorky, yet thoroughly charming and extremely good-looking Casey McCall. Who wouldn't develop a crush?