Questions and Answers
The explanation lies in the nature of arthritis and its most obvious symptom, chronic pain. The appreciation of pain has a very large subjective component, a component that may be influenced, to a greater or less extent, by external factors. Some individuals are more likely to feel benefit from a preparation with no pharmacological activity than others who obtain no benefit.
Perhaps the most important component is an unquestioning belief that the preparation is going to work, hence the importance of anecdotal good experience presented in a believable and persuasive manner. An exotic name (eg, Devil's Claw - very popular) helps a lot as does appearance and cost. None of this applies to your raisins, so your father's friend must have been well-persuaded him/herself and, to improve the chance of a favourable response, should be equally persuasive.
Best wishes to you and your father.