He needs to treat this very seriously because this sort of injury ruins careers - it's not hyperbole, it's happened to me and it happened to several people I know.
He needs to formulate a thorough and ongoing plan and stick with it.
He can self-refer to NHS physio. If he can afford it I would also 100% have private treatment too (which is far superior to NHS in this field).
Ergonomic desk set up. All of it, chair, desk, screen, mouse, keyboard.
Regular rest and gentle stretch/mobility exercise breaks, say every 30 minutes for 5 minutes. Yes it's a lot of interruption - but I cannot stress too much that his entire ability to do any computer-based job for the rest of his life, is at stake.
Focus his lifting on strong back muscles and actively reduce his much he does that's working his pectoral and other front muscles. Include pilates/yoga into his exercise routine so that he is lengthening and stretching out muscles to reduce nerve impingement.
Investigate dragon speech software and use it. It will feel frustrating and clunky, it takes time to get used to - do it anyway.
Alternate heat and ice on the affected areas.
Accept that productivity will reduce, and reduce his workload, before he has no option but to go off sick. Enough time off to rest and heal is the best treatment but as he is self-employed I assume he won't do that, so he MUST do absolutely everything else that he can to try to prevent a situation where he is forced out of his work entirely.