This has probably been said already, but I’m pretty sure Sam never actually charmed Weugan.
Weugan was acting as though it were on the outside of the spell looking in, noticing the strangeness of Evan and Jammer’s behaviour just as easily as Sam and K could.
Neither Evan nor Jammer were able to perceive their own nor each other’s behaviour as odd and were instead preoccupied with determining who got to be “the dog.”
The actual dog did not engage in their feud despite being the sole intended target of Sam’s spell.
Weugan didn’t listen to Sam because she had charmed it, it listened to Sam because she had increased the intensity of a truth about herself (and Weugan is all about increase).
When Sam started getting uncomfortable with the effect her spell had on her friends, Weugan implied that it saw no problem with it and suggested that the effect could last forever, assuring her that she’d done nothing more than amplify that which was already true.
Sam didn’t make Evan and Jammer love her. They already did. Sam didn’t charm Weugan. She earned its respect.
Sam said, “I’m worth listening to.” Weugan said, “Of course you are,” and listened.
Sometimes the scariest monster is the simple truth that you are already enough.