The callus that forms on a foot in someone with diabetes is no different to the callus that forms on the foot in someone who does not have diabetes. The cause of the callus is too much pressure on that area which results in the skin becoming thicker to protect itself. Eventually it becomes so thick that it becomes painful. The difference is that in those with diabetes, the consequences of that callus is much more serious with the potential for significant problems developing, such as foot ulcers.
Those with diabetes have some impairment of the nerve sensation, so the callus can become quite thick before it is noticed or has any painful symptoms. It could breakdown if not treated and be a portal for an infection developing with can go on to the serious consequences.
The treatment of a callus on the foot is no different to those without diabetes. It is best carefully debrided by a podiatrist. Paddings and foot orthoses can be used to help reduce the pressure that is causing the callus.
If you have diabetes and develop a foot callus, please get it treated properly and please regularly inspect the feet for any issues that may develop,