There are elements of awesome in that explanation... but the fail just out weights it. It would've been just as thoughtful to make Mercy a coyote-changing-person with no explanation and far less irritating.

...And Mercy/her mom still could've gone back to the tribe for help. Presumably that man had parents/siblings/cousins/whatever. And she would be close enough, blood-wise, to count on their rosters. There's still no reason for her to have gone to the werewolves.