Is there a regulation stating how an "off duty" reservist should talk to AD officers?

The thing to remember is they are average Joes just like you and me. Most if not all of them get tracer burnout from people trying to kiss their asses all the time. If you get a chance, discuss things NOT Military. Trust me when I tell you it will be a refreshing change of pace for them.

It is situational. In a workplace that you are both aware of your ranks it is professional and respectful to address as sir or mam, or by title. In social situations it's best to wait and see how the senior person addresses you and respond accordingly.

I served as active but many of our contractors were reservist in our reserve battalion. When it was drill it was sir or ma'am and by name when they were not on duty. But then again we used Mr and Mrs when addressing them

As many have already said, its situational. From a business protocol prospective; Is the AD Officer on an official visit, part of a VIP tour? Or is he/she just a regular customer frequenting your shop/office? In the 1st, addressing him/her by their official title would be appropriate. In the second, a simple Sir/Ma'am would be the best course of action. In either case (unless you are personal friends) a first name address is far to casual for a business environment. You can never go wrong with just being respectful and polite!