Aside from business rates and things like planning permission (or even operators' licences, if they're running a wrecker or whatever), he may contravening the terms of his tenancy. Whilst my landlord is happy for me to work from home on a self-employed basis (I do a little photographic work from time to time), I wouldn't be allowed to set up a professional, purpose-built studio at the house.
As the others have said, the local authority should be your first port of call, especially if they're upsetting the neighbours. Also, if they're causing obstructions, then report them to the police. When the people across the road from us left an untaxed vehicle half across my drive for a fortnight, I reported it as an abandoned vehicle. It was moved within 24 hours!