You don't need to have  masters  in finance or any other degree to know what to do. It's simple. Go to an insurance agency and ask there for a special insurance, for this case, because they have all kinds of insurances. At least give it a try!

Hi I had a similar problem a few years ago, unfortunately you can't insure something if it is already covered by someone else's insurance. BUT .... I was told by my insurance company, if and when i'm towing with my own 4x4, what ever I'm towing is covered through my insurance if an accident were to happen. If it was parked up ie on my drive/outside my house, it must have a hitch lock at least on it, but then it could be questionable as to if it were covered through my car insurance (but only if my car were to be stolen too) or my friends trailer insurance (if only the trailer was taken). If I were you, check if your friend has it insured, then check with your car insurance company because it may be that whilst your towing it will be covered. I hope this helps