I'm sorry, but with seizures this frequently your niece is not really dealing with the simple febrile seizures that children outgrow. This really needs more thorough evaluation and treatment. Perhaps her family can work with their pediatrician to contact insurance to get them to reconsider a second opinion; at the very least, they ought to return to neurologist #1 with this new information that the seizures have continued so frequently. That changes the approach.
Of the usual cake ingredients, the most typical ones that could trigger a hive reaction are egg, wheat, and cow's milk. Think about when he's had these items at other times, and if he's reacted to them.
Egg can sometimes fool you. Some people won't react to cooked egg, but will react to the uncooked (pasteurized) egg that's sometimes found in frosting.