I think they're as bad as each other and they're both to blame. See, all was well between them until Saanjh fell for Arjun. Now thats something that cant be helped, it was only natural one would eventually fall for the other and you cant help who you fall in love with.
Suddenly, things that once wouldnt mean anything to you and you'd laugh off, start to get to you. You become more sensitive to things. Arjun is naive and immature, hes always been that way, and for him the friendship is just the same as it was when they were younger. For Saanjh on the other hand, things are different now because she has feelings for him.
At the end of the day it comes down to honesty and good communication. They're the foundation of a good relationship. Saanjh should have been open and honest with him a long time ago and Arjun needs to grow up and be a bit more self aware and considerate.
Sadly, this is a one sided love. Even if Saanjh told Arjun how she felt and he told her he doesnt feel the same way, i very much doubt the friendship would have remained the same.