And if the pilot is simulating instrument conditions, there is a rule: FAR 91.109 "No person may operate a civil aircraft in simulated instrument flight unless - The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown".
If not simulating IMC, it's the pilot's responsibility to see and avoid per 91.113(b)
FAR 91 doesn't apply to military aircraft, although military rules require they maintain a visual scan outside the aircraft.
Civil aircraft means aircraft other than public aircraft.
Public aircraft means any of the following aircraft ...
(1) An aircraft used only for the United States Government ...
(2) An aircraft owned or operated by the armed forces ...
(3) An aircraft owned or operated by the National Guard ...
It's perfectly normal for a training, proficiency, or currency flight to be operated under IFR, even single pilot.
I suspect this crash is the result of a gap in FAA regulations. I would suggest that no pilot should be doing IFR training alone while around aircraft not flying IFR. But I can see why the rule doesn't exist: it is an extraordinarily rare situation that, probably, nobody thought needed covering.
This is one of the reasons that American's and Brits go to Goose Bay for low-level training. Canada has more empty sky than either of them combined.