is not his fault, and doesn't actually incapacitate him in any way.
He needs to commit to being responsible about it, he will always go to the doctor, take care for his blood sugar levels etc. When he is vigilant and responsible about keeping his diabetes in check, he won't keel over and die tomorrow. Diabetes is inconvenient, not life-destroying.
Practically nobody has 100% perfect health, nor do they retain their perfect health through their entire lives. Your mom is concerned for your longterm happiness, which is nice and all, but if she's saying this is a basis for dumping the guy, she's definitely overreacting. As long as he's committed to keeping himself as healthy as he can, there's no reason to dump an otherwise-excellent guy over his blood sugar levels.