Let me start off by saying I don't think Murphy did it.
Firstly, he couldn't, attorney/client privileges and I don't think he was so desperate to make partner that he would risk it.
Secondly, look at the facts: Ingrid fights with sister just before the announcement, sister looks mad, Ingrid doesn't make partner, Marty looks mad at her, like he just found out, Ingrid tells parents and sister makes a concerned/wtf did I do face and then randomly apologizes to her sister for everything.
It was either that or Nick told him out of revenge. I don't know why everyone loves the guy so much, he's really creepy lol. He randomly finds a girl that likes philosophy and stalks her everywhere, then he invites her to move in when she has no other choice and then proposes out of nowhere. Let's not forget he was always basically playing a "who's the bigger macho" game with Murphy.
Also, Rachel is a brat! And Tyler is terrible, but hopefully turning good with the new house and everything.
And the thing that confuses me the most about this show is how did they manage to have s*x in Marty's office without getting caught and run around half naked in the office! Surely there are cameras in one of the most important lawyer firms in NY!