In Chapter 22 of the book, Friday and another person are captives of the cannibals who evidently come to the island to have their festivities and then leave by canoe. One victim is killed, and the other, Friday, runs in the direction of Crusoe's fortress.
Crusoe had previously had a dream of this very eventuality some time earlier and is surprised that it is playing out in front of him. He convinces the victim that he is friendly, kills two cannibals who are pursuing the victim, and helps Friday hide in his cave.
Friday then claims loyalty forever to Crusoe.
This savage seeing a little chance for life darted away from his captors He ran very fast right toward my home I was dreadfully frightened when I saw this for I thought all the savages were after him...By a great deal of coaxing I got him to come to me at last He threw himself at my feet and placed my foot on his head to show me that he would be my servant forever I raised him up and treated him kindly <a href="http://books.google.com/books?id=amwAAAAAYAAJ&dq=robinson%20crusoe%3A%20how%20did%20Friday%20get%20on%20the%20island&pg=PA96&ci=204,771,722,282&source=bookclip">Robinson Crusoe By Daniel Defoe, Robert Trumbull, Samuel Taylor Coleridge</a>