I'm diabetic and have made a couple of sugar free cakes that turned out ok. They end up being a bit tougher or more dense than a regular cake. And they don't taste too good either, unless you use a lot of flavoring.
And depending on what type of diabetes she has, will depend on what you can make. Saying someone has diabetes is like saying someone has cancer. There are numerous types of diabetes, and each one has it's own "do's and don'ts". Some can have some sugars, some cannot have any, and others can't have sugars of any type of any kind.
Does it HAVE to be a cake?
I think sugar free cheesecake is disgusting, but a lot of diabetics like it, because it can work with pretty much any type of sweetener.
Sugar free muffins are pretty good, especially if you put a bit of fruit and nuts in them.
You might could get away with a type of sugarless Tres Leches cake. That would be a spice cake soaked in milk or cream, and you could sweeten the milk or cream with non-sugar sweeteners before soaking the cake. And then use the non-sugar sweetener in the whipped cream as well. I dont know about the fruit though.......some diabetics can't eat fruit because of the sugar content.
I don't know if any of that helps out, but maybe it will give you some ideas.