Could it happen that a diabetic live without a spleen?

Yes: Yes, he/she can live without a spleen. The spleen technically has nothing to do with diabetes and the body's ability (or, rather, inability) to produce insulin. However, since the spleen is heavily involved in the immune system, and diabetics are generally more prone to infections, a diabetic without a spleen would be even more prone to infection than one with a spleen.