Charging And Starting Systems

The other night me and some buddies thought it would be funny to move another buddies car about 50 ft on grass, just for the hell of it. The problem is it was 3am and we didnt have his keys. So we just hooked a tow strap to it and pulled it in reverse, while in park. He is claiming that i messed his transmission up so bad that every time he takes off it grinds. Is it possible to have messed up his tranny, even if i never got over 3-4 mph and it was on grass? I could see how it could if he was drug on cement or pavement. We did cross over 2 small brick walkways. My guess is he is just trying to get some money out of me. Please help!!