No, seriously: French fries should be no Problem. I don't know about the bun, but I assume it could be a problem and you should test it for yourself.
The meat should be fine if it's only beef, no pork.
When I go to McDonalds (which is not too often) I order only fries. I am gluten sensitive (so no bun), but I would also leave it because of the yeast. The hamburger is also not a very good option because ground meat breaks down very fast and is high in histamine. The only concern I have about the fries is that it's usually cooked in oils that I can't tolerate (sunflower). Sunflower oil has higher histamine levels....
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