In both cases he stated he was a fan of the people he was "reporting" to. However, your (EDIT: by "your" I mean Ute fans in general) only explanation/motivation of him being a BYU fan and seeking to discredit Spencer is the girlfriend rumor that Lucy's girlfriend started going out with Hadley. This rumor from what I can tell is completely just made up. Whereas the two statements by Lucy are not.
So, we come to motivation -- why would a BYU fan turn him in? Especially, late in the game, especially during rivalry week? The answer is that is it MUCH less likely than Lucy's OWN other claim that he is a Ute. The most likely scenario is that he came here to gloat under cover as a BYU fan so that his info wouldn't get discredited as much as it would have been had he been a Ute.