My older friend, who is a history PhD was convincing me that newest research indicates that the murder of Kirov, like burning of Reichstag, was not instigated by Stalin but only exploited for his goals. He pointed me to the work of Oleg Khlevniuk, as a definitive guide for the events.
Now I understand, that we might never be absolutely sure, but is there a merit to this argument? Is there any kind of consensus on the matter?
Your friend is not only correct, but the overwhelming consensus in current Soviet studies is that Stalin had nothing to do with the murder. This consensus is not even all that new; most of the pathbreaking research on the murder began in the mid-1990s. Matthew Lenoe's The Kirov Murder and Soviet History (which is more the definitive guide of the murder than Khlevniuk's works) appeared in 2010. In a podcast in the NewBooksNetwork. Lenoe was actually a bit embarrassed he wrote over 800 pages of text to explain how the murder was more prosaic. While there are some firm anticommunists/anti-Stalinists hewing to the "Stalin did it" narrative, the current historiographic consensus is that Stalin exploited the murder. For example, both Khlevniuk and Stephen Kotkin's biographies of Stalin are somewhat at odds in their interpretations of the man, but agree on this point. As Lenoe concludes:
Contrary to the denigration of the new archival sources by some senior scholars, new documents on the Kirov murder force us to revise received views of the development of the Great Terror of 1937–1938. The assassination remains a turning point in the chain of events that led to the Terror, but not because Stalin the master plotter ordered it. Rather, he used it. The murder frightened the Soviet leadership and probably Stalin himself. Stalin seized the opportunity presented by this fear to justify a nationwide hunt for counterrevolutionary terrorists, a hunt which he escalated steadily in 1935–1937.
So barring new evidence, this consensus will likely remain and while Stalin may be exonerated from Kirov's murder, neither he nor the Soviet state are innocent for their actions in the subsequent terror.