It's difficult to 'gybe slowly' in a real breeze, esp on a responsive boat. When the sail finally flips across there is likely to be some serious rounding up forces. I like to sheet in as far as I can prior to the actual gybe, and then allow the sheet to run in a controlled manner - not fetch up short. Your ability to be able to let the sheet run under control will depend on size of the main, number of parts in the tackle and the conditions themselves. Pretty serious rope burn can result of this goes badly.
The degree of round-up forces will likely vary with hull type and displacement. Obviously the helmsman and the main trimmer need to be in sync (as your 'helper' clearly wasn't!