When I was diagnosed with Celiac ... I too had done a lot of damage to my digestive system. In doing a lot of research it is an ALL the time thing. Your body sees gluten as poison and creates antibodies in your blood that will stay and do damage for MONTHS!! I would be wary of the doctor that said a small piece of pizza every once in a while ..
It is a lifetime commitment and can be very tedious but I find it so worth it all to not be sick and in pain like I was for years before ...
As for baking the cake, I know we are all about scratch but I have found that the Bob's Old Mill makes an awesome chocolate cake mix. I have a Paula Deen recipe that is gluten free using a cake mix. I just substitute in a gluten free cake mix and I am all set. I find the mixes in the organic/natural food section of the grocery stores here in WNY. If you want the recipe, let me know and I can send it to you ...
The chocolates I listed in my previous post are gluten free based on my research if you decide to go with the flourless cake recipe ..
Good Luck and I am sure she will be appreciative of all of your efforts ...