Pilot X is in an old bird and is not current on IMC flying so this is what he should do based on the scenario given.....
1 - Plan a VFR only flight with landing alternatives and VFR escape routes for the flight
2 - Definitely not press on hoping that weather is localised but have a clear strategy in mind for deteriorating weather and viz
3 - Execute plan B as soon as it look necessary
4 - Bear in mind that a precautionary landing (albeit with limited choices) may be preferable to dying by hitting the side of the valley
He / She should keep it VFR only and have sensible minima in mind which are more conservative than normal low level flying.
