I've been to multiple BYU games at ASU and the crowds have been mostly apathetic and certainly not hostile or unfriendly. There are a ton of BYU fans in the valley and there will be a lot of Cougar fans throughout the stands. The ASU student section will mostly likely be rowdy (drunk) so the language in that part of the crowd probably won't be good, and I would avoid those sections, but it's nothing like being a Cougar fan at a game against the Pukes up at Satan's Sandbox.