The holder of a UK issued PPL, flying VFR, has to be in sight of the surface (note: there is no requirement to see any part of the surface directly below you; the "surface" could be a mountain sticking up many miles away) unless holding an IMC Rating or an IR in which case he can fly VFR without seeing any surface.
The above is true
worldwide, regardless of the aircraft reg being flown.
The privileges which some license gives the pilot of a
G-reg are specified in the UK ANO. The ANO (Article 26, IIRC) limits the usefulness of an ICAO (non-JAA) IR to IFR outside controlled airspace i.e. IFR in Class G or F only. This is not very useful; you could fly IFR to Norwich (Class G) but not Bournemouth (Class D). And much of Europe is at least Class E.
If this chap is flying a Swiss-reg plane then the privileges of his UK issued JAA PPL will be according to the Swiss "ANO". I would guess that, Switzerland being a JAA member, he gets the usual VFR privileges, but he still has to see a piece of the surface somewhere.
He is flying in the canyons (most "mountain flying" is done inside canyons, it seems) but this
movie (17MB mpeg video) would generate a few comments about the pilot's sanity, I am sure
