If someone offers to treat for coffee, I assume they mean it, and get what I'd usually get. But I forego the muffin or whatever. Meanwhile, if I'm treating, the same goes for them--I expect them to get what they want, but not order a three-course meal while they're at it, you know?

Meanwhile, if it's a friend and we do this often, I follow their lead the next time.

Mind you, I don't know how people would intuit that set of rules. But it's like they say, "when you're lending money, consider it a gift." You offered, they accepted, it's a gift, go with the flow. Then tailor future treating opportunities accordingly.