Is it permissible to use the words "sadafah" (coincidence) or "luck"?

A believer must hold the conviction that everything that occurs in the universe is by Allah's will and decree. Allah Almighty says: "And with Him are the keys of the unseen; none knows them except Him. And He knows what is on the land and in the sea. Not a leaf falls but that He knows it. And no grain is there within the darkness of the earth or [anything] green or dry but that it is [written] in a clear Book." Indeed, Allah decreed all this fifty thousand years before the creation of the heavens and the earth.

So, if the intent of "coincidence" is the atheists' belief that things happen without the knowledge or planning of the Creator, then this is disbelief. However, if it means that something happens unexpectedly without prior human knowledge, such as suddenly meeting a dear friend, then there is no harm in that, for it does not negate Allah's knowledge and decree.