You are in an interesting position...they want to work with you. You have a product, users and serve the needs of this national chain. It sounds like you are asking for help finding a business model.
You will first have to answer some questions about your goals. Do you want this to be a full-time project or stay a side project? In most cases, you're probably thinking, "I want it to be full-time, so long as I can comfortably transition without affecting my lifestyle too much." That's probably not going to happen, but it might.
When negotiating from your position, you should be honest with them. If your goal is to build a system that supports loyalty programs for lots of restaurants, say so. If that's the case, you probably don't want to build out specific features for this client alone. Possible sources of revenue could be:
1) targeted adds - your app can either serve ads / push notifications to users (if you want to get fancy, you can use a geo-fencing tool to trigger offers). Everyone wants another point of contact with their users and they'll pay to get it.
2) License them the code and allow them to re-release it as a branded app...think of your current product as a white label. You can keep your app going and allow them to market their white labelled version and acquire their own users. This can include an upfront fee and annual payments based on the size of the user-base.
3) Sell them the whole thing...which they probably won't want to do.
Unfortunately, there are no standard terms in any of these sorts of deals. They want to work with you and that probably puts you in an advantageous position. However, it might not all that much in terms of what you can get. The person interested in incorporating you into the fold (somehow), probably can't swing a bigger budget than they have to work with and you're impact might not be big enough to warrant significant time and effort to hammer out non-standard terms.
Some things you'll want to ask for...and there is never any harm in asking are: ability / license to use their company name / logo / testimonials for your own promotional purposes, access to data w/ rights to use (in generic form), support agreement (x hours of custom work / integration baked in, with above and beyond as billable, but this depends on what kinda deal you strike) and...you get the picture.
Note, they came to you. Always start the conversation with, "Why are you reaching out / what type of deal were you looking for?" They'll lead and you'll follow.
I hope that helps. My email is in my profile, feel free to reach out.
