Markinabox;
The way you have described this incident is informing me that the end user has modified the rental forklift, and does he have a written authorization from the equipment manufacturer for approval to do this and to use the extensions. The regulation is as follows: "Modifications and additions which affect capacity and safe operation shall not be performed by the customer or user without manufacturers prior written approval. Capacity, operation, and maintenance instruction plates, tags or decals shall be changed accordingly."
At this point to remove some of the liabilty, put your concern in documented form along with photos addressed to all parties concerned to protect your employer. You also have a choice to logging a complaint with the Under Secretary of Labor for your district.