My DD is Type 1 diabetic and we adjust her insulin accordingly. She eats a normal, healthy diet and I just give her 'normal' cake if she's having something like this. The only thing you might want to do is be able to give them details of the carb amount in the cake (which will be difficult if you haven't done it before or don't know how to) in case they are carb counting. I don't agree with diabetic foods as such, the manufactured ones are expensive and taste like crap. I would just make a normal lovely cake if I were you.