I do think DD's enthusiasm for the game should be factored in. Obviously Bob is super into the game, but it's hard for me to tell from the descriptions how interested DD really was. Given Bob's level of enthusiasm and the fact that it was a championship game, I personally would have told myself, "This is not about me and Boyfriend getting together; this is about Boyfriend watching the game." Of course I don't really like sports at all. So a situation that resulted in Boyfriend being able to watch the game in a special, preferred way--even last minute, disrupting other plans we might've had--I think I would try to be a good sport about that, because watching the game was the main purpose of the event. I personally would even have been relieved that I was released from my "duty" of watching the game, and could do something else for the evening! I would assure him (via text) that we were cool, no need for him to feel bad, enjoy the game at the theater. In fact, please don't come over, because now I have to at least half-watch the game, and I can't read when the TV is on and would feel like I needed to be sitting in the same room the whole time, so it would actually be less fun for both of us.
But, there's a lot of factors to be considered. If I was trying to make watching the game at my place special, and had ordered/prepared food and decorations, okay then I'd be a bit more peeved that the plans changed. But, adding those extras would have been my choice and I think I would just have to, well, get over it, rather than blaming Boyfriend. I hope that if he knew of my extra planning, he would take that into account, though, and at least acknowledge that and apologize before going to the theater.
If it was something we were both really interested in, honestly I would probably be like, "There's seats left? Put a coat in one for me, I'm running over!" Because I would probably feel like seeing the game on the big screen was preferable for me as well. I think the toughest situation would be if it was 60% his interest, 40% mine--then I would probably really feel like I'd been ditched for a better offer, like my feelings about the game weren't worth thinking about at all because they didn't reach the same level of passion as his. And maybe that's the situation DD was in. As I said earlier, though, I think she should have cheerfully sent him back to the theater if she possibly could, faking good cheer for 60 seconds or whatever; if she could have predicted that she would be in a funk the rest of the evening that would just make them both miserable, it would be more mature and better for the relationship to short-circuit that. And then later they could still talk about what happened and how to prevent it in the future.