Can a person who was a boarder line diabetic 12 years ago be a kidney donor?

No: Probably not; but some variables may change such as tendency to diabetes rather than actual Insulin use. Some patients develop Insulin resistance due to massive obesity. Once corrected this is no longer a problem.

Yes: Diabetics can be cadaver kidney donors if they have normally functioning kidneys. It's not a good idea, however, to be a living donor because diabetes can attack your remaining kidney and accelerate your own path towards kidney failure.