There is no problem buying a coffee in McDonalds. I heard that straight from a very prominent Rav.
Here’s a funny story: I was once driving from Canada to Brooklyn overnight. At one point, I stopped at a rest area, and wanted a coffee to help me stay awake. It was about 2 am, and the McDonald’s at the rest stop was the only thing open. I went to the counter and asked for a plain black coffee. The guy behind the register looked at me, looked at my kippah, and said, “I thought Jews aren’t supposed to eat at McDonald’s? We’re not kosher!”
I explained that plain coffee was fine, and he was glad to learn something new.