He needs to register via URSAFF (think - there are links on other threads) as a “posted” worker - basically become PAYE in France via the UK company. I assume from your wording he runs a small Ltd company. Word of warning - minimum wages apply so no I pay 200 salary and take the rest in dividendswhen it comes to registering
Or shut the UK ltd and set up as Autoentrepeneur in France - or mix and match if he’s enough clients and some work is done in France and some outside France - if he WORKS in France its one or the other - if he physically returns to the UK to work then he can stay as Ltd - pay UK tax as always and declare wages/dividends in France - with additional social charges to pay.
Give Brexit isn’t finalised its hard to know - tax won’t change but cross border business may be interesting.