This time it is only fair that I share the driving in Thailand with my Thai friend using their car.
The main issue that I see before my trip is car insurance cover. If I were to get a hire car then the car insurance (policy coverage choices) would be part of the cost. Perhaps, I can get some cover by telephoning the company I bought my annual travel insurance from.
How many Km do you plan on traveling in total? Over how many days? Unless your friend dislikes driving I'm not getting the need to share? In fact, it might be a detriment, not a benefit. Your friend is accustomed to driving here, you are not. Maybe you could wash the car? Pay for gas? Deal with maintenance and repair?
And what forms of accidents are you looking to insure? Automobile? Personal/medical? If the former, your friend has insurance. If the latter be aware that some insurance may not cover you if you are in an accident here.