If he insists on applying a finish maybe for looks or to make the surface smoother a boat varnish from a marina would be best because it would hold up better against the sun s uv light.
Best wood bench finish. Dyes impart some color to the finish but do little to block uv rays. Build the finish until you like the look and enjoy your workbench. Once damaged wood fibers separate from the underlying wood taking the finish with them. Film finishes especially better quality ones that flex after curing best protect wood from moisture.
You can t caulk a bench like you do the framing around windows. The most durable and the most labor intensive natural wood finish consists of three coats of epoxy followed by an exterior varnish topcoat. The epoxy provides a moisture barrier that prevents. An epoxy sealer with an exterior varnish topcoat is the most durable outdoor finish and can last for many many years.
What kind of finish would you recommend for a workbench top made of softwood. Otherwise skip to the next step which is to apply several coats of oil based gloss polyurethane. If that is the case seal the wood with one coat of de waxed shellac or zinsser sealcoat. An exterior oil finish is definitely the simplest quickest way to treat an outdoor project.
I would recommend a drying oil finish something like a watco danish oil. However the initial application does take longer then exterior varnish. Best finishes for outdoors. Common brands include interlux epifanes pettit and z spar.