^I think that is the most reasonable approach if he had time to keep riding after the 4 weeks.

Sounds like after 4 weeks of available time, he'll have to taper off again. I'd think just getting out and riding unstructured and enjoying yourself and listening to your body would be best. Use that time to get comfortable riding again, maybe build up a small nugget of something resembling endurance, then buy a trainer and kill yourself on that instead of the rowing erg.