It's a tricky one with regard to planning law.
In general, it's OK to use a caravan or motorhome, parked on your home premises, as an annex to your property. (e.g. as an office. or even as an extra bedroom). However planning permission is required to use it (for longer than 28 days) as a separate residence.
Your friend's suggestion seems to be on the borderline of the rules. i.e. if he's inviting the person he knows into his home, and only using the motorhome as an annex, the local authority MIGHT consider that no 'material change of use' planning permission is required, as the motorhome is not being used as a 'separate residence'. However I suspect that someone locally would object, forcing the local authority to review whether there has been a 'material change of use' for the land upon which the motorhome stands.
Chris