The final line of "Dear John Wayne" can be seen as referring to John Wayne, Native Americans, or both.
Part of the reference is clearly to cancer, and the metaphor is that colonialism or imperialism or land greed is cancerous or cancer-like. Cancer was what killed Wayne. A further irony is that he likely acquired it from radiation from nuclear testing that was going on nearby when he filmed The Conqueror. In it, he portrayed Genghis Khan, with yellow-face makeup and eyes taped back to fit preconceptions of Asians.
Most of Wayne's films, especially his westerns, express a belief in Manifest Destiny, that the lands of the US were destined for colonization. Wayne also called himself a white supremacist in a notorious Playboy interview and felt Native Americans were "selfish" in trying to keep their land. The poem's author, Ojibwe poet Louise Erdich, could be arguing Wayne and others who covet Native lands will die by their own cancerous mentality.
Another reference in the final line is skins "splitting out." "Skins" is a Native nickname for themselves, a shortening of the epithet "redskins." It is an attempt at taking away the power of an epithet to harm, much like blacks using the N-word among themselves. Erdrich could be arguing that Natives will reassert themselves. Or she could be saying they will abandon their heritage.
I'd argue all three interpretations are potentially valid.