Great tractor! It is a real workhorse. And you can't beat free. I've had mine since 1977 and plan to keep it running indefinitely so I can keep using the snow thrower. I've been looking for a parts tractor and haven't found one under $500 in RI or MA even without any accessories. If I were you, I'd take your friend and his wife out for a nice dinner.
One suggestion - while you have the carb apart, make sure you check the float. The original fiber floats crack and stick - causing problems from flooding to no flow at all. If you have the fiber float, replace it with a brass float (about $20) even if it isn't yet cracked. You might want to replace the high speed jet at the same time (about $10).