Hey, I dont thinks so its Ishaan fault, its actually natural anyone would do that, Radha Bua is like a mother to Ishaan, and Sushana has just entered into his life, Ishaan loves both the ladies, and he known's that its not Sushana fault at this time, but he cant say anything to Radha Bua, coz she loves him like his own child, which he respect her for that. and i think the reason he left the room, as he know that Sushana is very angry and she wont understand his situation , he can also stop Sushana not to be angry at Radha Bua as Ishaan knows that its Radha Bua's Fault, but i am sure he is going to talk to Sushana that he understand he situation also. Also Ishaan is not supporting Radha Bua, he is also worried for her, he loves her too.
Honestly speaking in real life if u love two ppl who you love, its hard to pick one of them especially when there is an old relationship and a new relationship.
I think that Ishaan is very right in what he is doing. i support him, I know he loves sushana alot and he will convince her and also he will tell her sorry too, even though its not his fault