Your friend would only be covered to drive the horsebox should they be named on your policy or you have an ‘any driver policy’. This can have restrictions so check your policy wording. For example, it may only cover drivers over 25 years old. Even if they have their own car or horsebox insurance, it would not cover them to drive your horsebox. In terms of trailer insurance, the trailer itself is covered no matter who is towing it with the owner’s permission providing they are insured on the vehicle they are towing with. This would be covered when attached to a private car under the ‘towing risk’ section of the client’s policy wordings or it could be specified on the private car policy.