I've slipped up and eaten off plan recently and let me tell you it was a big mistake! I don't get cravings for sweet/startchy foods anymore, but when I had a small bit of cake I just couldn't stop and I ate and ate and ate until it was bedtime. I don't know what I was thinking at all!!! I had lost control. If I was you that's the sort of thing I would be concerned about. Why don't you ask them to bring strawberries and cream or something low carb instead of a cake..
Whatever you decide best of luck and happy birthday