Nevski appeared in the flashback in the level "Project Nova". He assisted Reznov and Dimitri in eliminating the last German forces, but was betrayed along with the others by Dragovich.
Belov, Petrenko and Vikharev are thrown into one chamber and gassed first, while Nevski, Reznov and Tvelin are put into the adjacent chamber. The arrival and attack of British commandos allows the trio to escape from their chamber and fight their way out. While Tvelin was killed, Nevski and Reznov manage to escape the ship before it explodes from Reznov's explosives. It is unknown what happens to him after the incident.
If the player watches Nevski after they escape the ship, they will see that Nevski continues running until just before the end of the flashback, when he stops.
When he stops at the end of the flashback, Nevski stands in a normal position as if he's holding a weapon, but he has none.
Despite Petrenko telling him to grab the gear, Nevski never grabs anything, instead he just gets up and walks to the truck with Reznov and Petrenko. However, he always carries a backpack with him, so it is possible the "gear" is survival equipment, such as tents or food.
He runs off into the horizon at the end of Project Nova while Reznov lies on the ground recovering from the explosion, suggesting he may have abandoned him. If not, he and Reznov might have stayed together until Reznov was captured and sent to Vorkuta.