I don't have a clue really, but I have had a few ride on mowers so I can use logic.
We had one that was designed to pull a roller filled with water, a three gang mower, or a barrow. None of them was heavy, but I would be inclined to find out the weight of your mower and only pull half of that, Does your friend use it for towing at the moment? I'd worry it might tip up with too much weight behind it.