I do the same, a friend rents one of my cars.
I just cover 'other drivers' on my MSIG Insurance. They don't have to be named, but can if you want to.
Have done this way for years...
So to the OP...you don'have to do anything, other than make sure you are covered on your friends insurance policy (and he should have 1st Class Insurance, with the Compulsory Insurance coverage paid also--645 baht a year).
Happy driving...