I am also very glad to hear that you got it back!
In considering that it may have been ridden in the rain without an air filter on it, dirty water getting slung up into the carb, and on top of that not having pre-mix fuel ran through it
, I definitely would get or make a new base gasket and pull the cylinder and inspect both the cylinder and piston for damage.
The sad part is if any grit got inside the crank bearings and con rod big end bearing there is no cheap/ easy way to inspect them.
Regardless, with it still running or not, I believe that he owes you a whole new engine due to the fact that he abused your current one. By simply removing the only protective device the engine has from the carburetor (air cleaner) warrants the notion that damage has been done. The gritty/ sticky carb action definately suggests that dirt has been ingested and is trapped in between the throttle slide and slide bore. Also if I read you right, he admits to using straight gas , no pre-mix oil?
That will absolutely ruin a 2 stroke engine!
I am not recommending a law suit here, that is your choice and yours only, but you certainly have grounds to do so.