Take care of borrowed equipment better than yer own.... that's what my daddy tought me. Had to borrow a backhoe to dig a water line trench this weekend... guy fueled it up and said its ready to go... he takes good care of his stuff... I'm gonna go over it with a grease gun after I fill the fuel tank before I return it.
Hope you luck out with the repairs to the brush hog... I agree it looks like the clutch not the gear box. Might not be cheaper to fix but maybe easier... no crawling under it! Good luck... be straight up with the owner, they will likely help you out with labor and/or cost... honesty goes along way.