Mayank had gone too far with his 'you manipulate everyone for your own purposes' comment to Nupur and so she lashed out at him as well.
Maybe she could have avoided making such a mean spirited comment such as 'you dont' have any friends'....but then, it had all come down to the fact that she wanted to hurt him as much as his harsh words has hurt her....so that's exactly what she did.
I thought it was a BRILLIANT moment! I totally enjoyed Rati's acting in that scene.....
From a storyline perspective, it was necessary....for Nupur to finally have enough and stop apologizing to Mayank.....now they are both going to go out of their way to be rude to each other....and I'm loving it😆