What affect does orlistat have on a diabetic?

None. It only inhibits the absorption of fat by inhibiting lipase which breaks down fat for absorption.
Some benefit. For the most part it would have the same effects as without diabetes. That being said, weight loss is a huge benefit in diabetes and can dramatically improve the control and prognosis of the disease so if orlistat helps you lose weight , it can only help. Orlistat can cause bad diarrhea and if you have a tendency for hyperglycemia use caution. Always check with your doctor first!