I dont think Yash knows about Prashant because Artis friend said she left the message on the phone on 10th august. But yash as soon as he woke up on the 10th of august he packed the bags and left for bhopal.
And I dont think artis friend called before they left as she left the voice message on the phone since the call was not answered. This means she has called after they left for bhopal.