Unfortunately for him, the Sole Survivor kills both of them and is able to retrieve clues about him. Vengeful, he personally captures Kent in Goodneighbor and takes him to his hideout, the Milton General hospital. He holds him captive until he is killed by the Sole Survivor.
There are a few ways to save Kent. One is to speak as the Shroud to scare away Avery and Sinjin's lackeys, then quickly kill Sinjin before he can kill Kent, to convince him to let him free and blame all on the Sole Survivor with a high Charisma, or convince him to attempt to kill the player character first, and use jet to injure him and Avery. Alternatively, one can quickly kill Kent using V.A.T.S. Doing so will confuse and terrify Sinjin as well as Avery and the lackeys.