Prashant isn't her driver, she should not be sitting at the back. That would be so disrespectful... It's late at night and Prashant's taking her home. If he wanted, he could have just left her there, but he didn't.
He hasn't done anything or said anything to ruin Aarti and Yash's relationship. The only time he "interfered" was to make it better... and I'm telling you he succeeded. He was trying to make Aarti the only focus in Yash's life, just like Aarti made Yash hers when he was talking about Arpita's picture. Prashant realized how devoted Aarti was to Yash and tried to help her out.
Don't raise your blood pressures over this, because Prashant hasn't done anything he shouldn't be doing.