Why is this even a question? Last I heard, Mickey D's is not a government agency, so there's no question at all. They can put up whatever sign they wish.
This woman has no understanding, apparently, of the Constitution.
Now, she can, if she wishes, march up and down the public sidewalk near this place and picket it as long as she likes. That's the only appropriate response available to her.
Of course McDonald's in general can put up whatever signs it likes. But all McDonald's outlets are either corporate-owned or operating under strict franchise agreements. Individual store managers and franchisees are certainly prohibited by company policy from posting such sectarian messages. I can guarantee that any McDonald's franchise that sprouts an "Allah Akbar" sign will lose its "McDonald's" sign within 24 hours, and any corporate-owned McDonald's that sprouts such a sign will have a new store manager within 24 hours. Corporate management may look the other way when it comes to violations involving Christian messages posted on stores in overwhelmingly Christian areas, but once someone lodges a formal complaint, I expect policy will be enforced pretty promptly and the sign will come down.
To the Left, anyone who "offends" must be silenced, the quaint notion of "Free Speech" be damned.
That is why Canadians are so satisfied with their self-stifling "hate" laws.