I imagine there were a number of nuclear war missions planned during the cold war that were not survivable -- manning this bunker being just one. That said, in the here and now with less than full-blown nuclear war in mind (think EMP/dirty bomb) a cold war bunker may be ideal. NBC filters are a good start, but if your friend has any interest in gardening, a greenhouse or storage shed built over the bunker could provide support to the bunker and could be very useful even without the bunker. Whatever, those are my thoughts and may not be of interest or within budget. Have fun.

When I put not survivable above I meant that the posts were designed to increase chances of surviving not guaranteeing survival.

They would easily withstand a 10PSI overpressure and the accompanying blast. Fallout PF was about 1000 and particle protection was perhaps three days. All enough to do the job and probably good enough for the great majority of observers. a closer detonation or being in a rained out fallout plume would have been a problem.

spending more money and installing more kit would not have changed the odds much, getting into the area of diminishing returns. It is difficult to define the line for such kit.

There appears a common misconception that nuclear war would involve many weapons, in reality a few would probably be enough. In the last nuclear war two weapons were enough, and one of those was superfluous and just used as a weapons test.

Since these bunkers were (fortunately) never used during an actual nuclear strike, there is no empirical track record on which to base improvements. It seems prudent that if you are going to rehab one, it makes sense to add air filtration just to mitigate particles and NBC contamination/agents finding their way into the shelter. The concrete structure under 20 feet of soil should do a good job protecting the occupants from Gamma radiation, but it seems that the unfiltered air would allow other contaminants into the structure where they could be ingested. Just my opinion.