Nick proved that he cares about Native Americans more than any other character in the story by crying at his girlfriend’s infidelity. Because of this (his concern/affection), Nick symbolizes the tenth Indian.

or

The fact that Nick cries about his girlfriend’s cheating,symbolizes that Nick is the tenth Indian because he shows that he cares more about the Native Americans than any other character in the story

The fact that Nick is affected by his girlfriends infidelity proves his affection for Native Americans, more so than other characters in this story. In this way, Nick can be seen as the tenth Indian.”

Nick is upset that his girlfriend cheated, which shows that he cares about Native Americans more than the other characters in the story; this affection makes Nick the tenth Indian.

Nick can be seen as the tenth Indian in this story; he cares deeply for the Native American in the story. This is in sharp contrast to other characters in the story, and is witnessed by Nick crying when his girlfriend cheats on him.