If a product is already in the market for a specific purpose and is found to be useful in another function by someone else can that person apply for a patent for that new function?
As a patent is a territorial right you may want to clarify where protection is sought/ which national/ regional legislation should be applied. Your pofile does not state your origin. At this time this may help regarding EU-related matters:
The question cannot be answered definitely but one way to get protection may be a use claim. Consult a lawyer of your choice and provide him with details.