At least the exterior shots of it.

I don't think so, but I'm getting a bit rusty when it comes to HD...

I think that there actually WAS a real diner that they used, but it was an old closed-down one. It was based on a real one though, I've been searching for it today, and I found this:

The Cunninghams' House
Fictitious Show Address:
565 North Clinton Drive, Milwaukee, WI
Real Address:
565 N. Cahuenga Avenue, Los Angeles, CA

After a tough day, don't you wish you could drop by Mrs. C's for a good dose of motherly optimism? Well, you can't burst in like the Fonz used to, but you can still drive by. 522 North Cahuenga in Hancock Park was the distinctly Midwestern exterior of the Cunningham family's home on Happy Days for over 10 seasons. The illusion was seamless on television, but you'll know this Wisconsin look-alike is a California native when you check out the landscaping in person. Richie Cunningham's old back yard is full of palm trees.

http://travel.discovery.com/fansites/roadtrips/facades/facades_02.html

I was just reading where a place called Kopps Frozen Custard in Milwaukee was used for the exterior shots for Arnolds. They have their own website, but no pictures of it.

i dont know it probably was an old restaurant closed down