The reason that a working on a diabetic person can be a concern to the nail technicain is not to do with wether or not the nail enhancements will stay on, but rather the physical condidtion itself can be a cause of poor circulation in the extremities (hands and feet) which can affect how quickly a person heals if the soft tissue surrounding the nail beds (fingers or toes) is abraded. If healing is VERY slow, then infection is a cause for concern.
Most diabetics these days are well under control with medication and it is generally not a contra-indication to performing a nail service. You can work perfectly happily on a person with diabetes but do a thorough consultation as always. The client will certainly be well aware of her condition and will be the first to tell you if she has concerns.
If you feel you must get the doctors permission, there is a very good sample letter under the NVQ section on the www.designernails.com web site.