I don't know why you want to shake them both. I see Tina doing no wrong in this situation. She makes a cake and has a party that she has had for years now. She offers plenty to eat for all of her guests from a varied menu.
Tom is being a jerk. Tina offered to get him a cupcake from a bakery with his dietary restrictions. She has other items on the menu that Tom can eat. He wants none of that. He wants Tina to change her grandmother's cake recipe, (a tradition that Tina has made for a few years now), to fit his dietary needs. What if the cake then does not meet another guest's needs?
This is not about Tom's diet. This is about what Tom wants.
Personally, if I was Tina, I would be telling Tom, look this is it. At this time, I am not even going to offer to get you that darn cupcake. If my annual party is so much of a problem for you, stay home.