What to use to cut things on non stick surfaces?
I'd pre-cut them myself - I tend to cut mine in the tin using a plastic spatula; after all they're soft and easy to cut. Then

There are stiff plastic knives (for some reason sold for use on lettuce). They're much better than metal and will easily cut brownies.

If you did want something disposable, some of the wooden disposable cutlery is surprisingly robust, and should be safe on non stick. As you're reckoning on getting the tin back, I'd hope you'd get the knife back too.  There are also semi-disposable plastic pie slices/servers (eBay link) but I haven't tried them  myself.

You can't just use a ceramic knife. It's harder than metal and just as sharp, and the problem is both, but especially the hardness. Even a table knife or the edge of a metal spoon can damage a non stick coating.