If I have to be somewhere at 7:30, then at 6:30 I'm in the shower. And at 7 I'm running behind in a frantic. Who knows her routine. It's possible that it happened after she left, or before. Either way it's possible IMO without having her be in on it. I think he is asking because he wants to make sense of it, and it would be easier to think of someone else as being guilty than a woman he was dating.
I personally think there would have to be evidence to believe he is involved. I don't, right now, believe that at all.
Personally, even hardcore gang-bangers would have been repulsed by this. 2 months with this girl is not enough. This, imo, has to be more personal than that.
There was a study in 2010 (Diem & Pizarro I believe) that, if i recall, even showed that the community/familial makeup of American Latino culture make familial homicide even less common. Just being immersed in it myself, I just highly doubt he is involved- not without some damning evidence. Police believe him, so I do too. Unless that changes.
Besides, he'd never admit to being where Tyler was later found unless it was already "out" and he was defending himself. He volunteered that. I don't know. I don't see it.