Was Frederick Nietzsche the first to use it in his 1888 book Twilight of the Idols?
I no longer think so after listening to a provocative guide at the partly excavated ruins of the San Isidro de los Destiladeros sugar plantation and mill near Trinidad, Cuba. She described how slaves barely got enough food to stay alive – owners provided just enough to keep them working. When they could, weak and starving slaves foraged in nearby woods for anything edible. But Cuba's plants were completely unfamiliar to the Africans. Some of the collected plants were edible and "made them stronger." Poisonous plants killed them. In Spanish, "Lo que no te mata te hace más fuerte."
From Encyclo-Britannica:
In the 19th century Cuba imported more than 600,000 African slaves, most of whom arrived after 1820, the date that Spain and Great Britain agreed would mark the end of slave trading in the Spanish colonies… in 1860 Cuba produced nearly one-third of the world’s sugar.
Best Answer
That is at odds with Nietzsche's expression. Nietzsche is referring to hard experiences that could kill one making one stronger, if they don't.
What she describes predates Nietzsche, but her describing it was after him, so she could well have been influenced by him in turn of phrase, even if the meaning is not the same. Either way, someone using a phrase about something in the past does not tell us anything about the origin of the phrase.
There's nothing here to suggest we shouldn't credit Nietzsche with the expression.