Your friend is liable. No ifs, no buts.
From the other post it sounds as though he had no insurance for the buggy. His household insurance MIGHT pay as suggested there. But most household insurance only covers for issues arising out of being a houseowner, not for driving vehicles.
The disclaimer form is irrelevant. It would not have changed the liability in any way - just pointed it out.
Unless he can prove the golf club were responsible for the accident - ie the buggy was faulty - he really has no way of shuffling liability off onto anyone else.