Abhi said that Raj has a very powerful background. No one in armed forces can defeat him. Moreover we all know he is really talented ans strong. Then he cud have saved Navin in some or the other way.
Raj himself said to Naina that "In chaar saalon mein Navin ke maoth ki sachhai ke saath har ek pal jiya hoon." So even after 4 years of hardwork, he wasnt able proove that Navin was innocent.
Raj also said Abhi has been trying to solve this case. But he also cudnt solve it in these many years..So awkward!!!
If Naina hadnt taken efforts( with whose ever help it may be), the case wudnt have reached this level. Being a girl and without knowing much abt army and just with martial art training, she has come to KMA for her bro. Hats off to this girl!!!