This technically wrong. The exemption from the Gas Safety (Installation & Use) Regulations (GS(I&U)R) identifies touring caravans, and self propelled motor homes, that are not owned by or or used for any business purposes, are an exempt from the GS(I&U)R and the owner may undertake work on the gas installation and appliances.
The exemption specifically only applies to the private owner of the specific vehicle, thus it is only the owner who is exempt, if anyone other than the owner works on the gas system or the appliances they legally must comply with the full GS(I&U)R,
If the owner askes or contracts anyone else (inc. relative, friend or contractor) to undertake work they must have the required qualifications and ticket to work on the system. The Owner will be classed as the "Person with responsibility for the system" and as such should use due diligence when selecting a company or persons to do work on the system.
It should be borne in mind that the GS(I&U)R, is part of the health and safety legislation, and failure to comply is classed as a criminal act and if prosecuted can be subject to fines and or custodial sentences.