AGCS radio (I know it is not quite the same), according to CAP413 Chapter 5.2.3 does not need a view of the airfield as all traffic information passed is based on position reports from other aircraft.
However if said operator does have a view of the airfield and he does see an impending incident he would act under his duty of care.
Having the benefit of windows does show up the limitations of the usefulness of the minimum standards required of an AGCS or FISO at airfields.
D.O.