Siddhart didn't talk to Amar first, Amar and Akashcheap's group ditched each other which left him on his own too. So whose the obvious choice to talk to? Yup, Siddhart. And it's not about his asking for support or not, in his support Siddhart made some solid points which back-fired against Sunny while trying to assasinate Amar's character, and at least for that reason there was no need to turn the table against Siddhart for Akashcheap's sake.
As for Mahek's lie, instead of listening to Akashcheap's insults and even apolosing for it, why the hell didn't he confront Mahek about her 'lie'!? If he did it for the sake of 'game' or because he, so-called, didn't want any 'trouble' then damn, the guy indeed is spineless!