the kid goes.
I did my undergrad (pre-mission) at BYU, came back and went to the Air Force Academy, then PhD at Georgia Tech. My dad never stopped rooting for Utah, but he was absolutely supportive of everywhere I went and proud of each of those (even BYU which was a fallback choice for me).
If the family isn't BYU grads, and are primarily BYU fans by reason of being LDS, it makes perfect sense to me for them to start celebrating Utah. That's where the kid is going, you want Utah to do well and you want to support it. Especially (in this case) when it wasn't the first choice and the kid probably already feels pretty bad about not getting to BYU.