He really should tell insurance company, as if they want to be akward they could refuse to pay out on a claim.
I have added wide wheels to a Golf, and reckon they make car safer, but insurance want more coz in their eyes it
means you are more likely to drive fast, push car to limit, and have crash / make a claim.
Similarly adding twin exhausts shouts "boy racer and higher risk" to them.
Maybe just put it back to standard, as once insurance told they may refuse to deal with him!!!!!