Kaja On the one hand, that seems like an event where wearing the belt would be very useful. She is allowed to say no, but there is a decent reward for saying yes. On the other hand, it also gets in the way of chaste activities (exposing her belt to the masses, like most swimwear would do, is not something that should be expected of her) and it is an additional expectation on top of her original agreement that she is not given time to prepare for and weigh the pros and cons, or nehotiate alternatives for that matter.
I would say go to the party belted, but try to arrange a method that allows for you to enjoy the beach in daylight while still being protected at night. Maybe your mom will accept a compromise with tamper seals or something.