The thing about Raj is that he was always on the mark when it came to Armaan Jassi relationship. He was always shown watching their interactions and accurately assessing them. He first pointed out Jassi's feelings for Armaan, even when Armaan would just brush it off as nonsense, even before Jassi admitted it to herself. He would have to point out to Armaan to be considerate to Jassi - first as a human being (remember the incident when Jassi came to the office limping and Armaan did not even realize she was hurt, instead kept yelling at her?); and then when they made their "plan" he had to practically force him every step of the way to go thru with it. From that to Armaan expressing his concern for Jassi when Purab left and his guilt after finding out about Samir, grabbing Raj's collar when Raj disrespected Jassi. That is a huge change.
Armaan may pretend his concern (though why now???)with Jassi, why will he pretend about Jassi with Raj -the one person in front of whom he lets his guard down? If anything, his denial of any true feelings for Jassi in front of Raj would be understandable, not faking a concern he doesn't feel for her.
Raj has always been very astute when it comes to Armaan's feelings. He may not understand how a guy like Armaan could ever come to care for someone like Jassi, but don't tell me that he has not figured it out.