Thing is, there are certain types of separation that will require navigation by visual reference (say, geographical separation) and in those situations, the pilot should be able to establish themselves by visual reference to one side of the (usually) line feature described. Or visually remain in a particular defined sector of airspace.
You wouldn't know if they are using GPS/ RNAV as an adjunct to this, of course. If they were solely using a navaid-based source to stay in a VFR sector, that would be invalid. But you probably wouldn't know.
There is no reason a VFR aircraft should not fly final approach via the ILS (traffic permitting) provided they are doing it in VMC, and capable of continuing to navigate visually. (T'would be sad if someone on an ILS final in VMC got lost if the ILS was turned off on them. But then, I've heard of some trainee pilots declare an emergency because the artificial horizon failed on a VFR nav-ex.)