Somehow I don’t think that old “your honour, I only sold him the gun and what he did with it after was his sole responsibility“ would hold much weigh in a court of law.
Such would also be the case when shops perform illegal modifications but to be held to account, it would require an interesting case to wake the DPP from their slumber and get them to actually apply the law.
As is, businesses have specific responsibilities in law and their liability related to the mods that they do is greater than many seem to appreciate or acknowledge. Frustratingly, they all seem to understand their liabilities when it comes to you wanting to check the work they are doing and inspect the vehicle while it is on the hoist - can’t do that cause of OH&S bullshit that is always referenced… Hypocrisy thy name is business
Kudos to him for doing things correctly and by the book