I am not aware of anything that points to her being dead that afternoon. That is wishful thinking on your part.
There are however things that point to her being alive that afternoon. Such as the eyewitnesses, the earing, the cell text, etc. etc.
You say there was some sort of argument the night before (even though you say you didn't know her, so you are getting your info second, third, forth, seventy fifth hand?). So what? That doesn't mean she was killed that night.
Think about it - if someone supposedly witnessed this argument, and feels the need to bring you into the loop (but not LE), don't you think they would also have told you about witnessing her being killed as a result of this argument?
Or are you suggesting that there was this argument, everyone went home, and then she mysteriously died without witness or trace for no apparent reason?