I heard that yes, you are allowed to buy the coffee. But no, you are not really allowed to do into those places.
It does not look right, and more importantly, you are putting yourself at risk since McDonalds is known for its junk food with a strong smell that makes you want to buy their stuff.
It’s kinda akin to going to a church for some matza because they are the kind that like to have a little seder around Pesach time.
My thought is that our precious neshamas are worth not putting at risk for the extra 37 or so cents for coffee.