So there were two different segments regarding Roshni being thrown out...well I have a thought.
The thought is...When Simran tells Sid what happens he does stay silent unaware as to what to say. Seeing him silent Roshni feels she does need to leave as maybe that is what Sid wants. Then she begins to leave & one of the guests says something ignorant about Roshni and then Sid bursts and stands up for her...I feel when Sid stays silent was not that he was not supporting Roshni bt I think he was just shocked and trying to digest what he just heard.