Your friend goes back a ways in terms of Buff history; the last “true” tail gunners were in the D models, which were retired in the early 80s. I’m assuming he probably spent some time in the G/H models, where the gunner was moved “up front” with the EWO.
Incidentally, D-model gunners were responsible for shooting down two North Vietnamese MiG-21s during Linebacker II missions in December 1972. The first was credited to SSgt Samuel Turner, the other to Airman First Class Albert Moore. Your friend probably knew (or knew of) both; Turner was a MSgt who was still on active duty when he died of cancer in the mid-80s; I don’t know what happened to Moore.
During one of my Air Force assignments, I worked with a former Buff gunner who became a training manager after the gunners were eliminated from B-52 crews. He began in the D model, but transitioned to the H model. Said he much preferred the D, because his “office” had a window in that model. With the lack of an outside visual reference, he said that low-level missions were tough in the H and even worse for the nav and radar-nav “downstairs”