Bottom line - unless he was effected by an emergency in which case I'd go easy on him, unless the airfield is ATC controlled he can't do a "land after" - in fact he's not allowed to land whilst you're still occupying the runway, which you are if you are backtracking and that's the only realistic exit option. Now I'll admit that there's a world of difference between the law and practical reality when faced with this situation, but I'd have certainly not done anything that would have risked my aeroplane (like taxiing off into the grass) and would have probably said "G-XXXX still occupying the runway suggest you go-around". And if he still landed, a report would have been in order.
Andy