What's the Name of That Book??? discussion

This topic is about
Keeping the Moon
SOLVED: Children's/YA
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SOLVED. YA romance/drama from c.2000 - teenager whose mother is a famous fitness instructor goes on a beach vacation and befriends two waitresses [s]
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I've long since lost or given away my copy and it's not a book I'm particularly wanting to re-visit, but something reminded me of it the other day and since I'm unhealthily driven to know exactly what books I've read if at all possible, I thought I'd post here to see if anyone else remembers it.
Plot details I can recall: it starts with the protagonist, who is in her late teens, being dropped off at a bus or train station by her mother (who is a famous fitness instructor or weight loss guru) and her mother's PA. The girl is going on a beach vacation on her own for some reason. It's mentioned that the girl used to be overweight until she became her mother's test case for the fitness regime that's made her a lot of money. When the girl gets to the vacation spot she quickly gets friendly with a couple of waitresses at a local cafe who are around her age. One of them is kind of a party girl, the other's more shy and serious and is engaged to a guy who her friend considers pretty seedy. The strongest recollection I have of the book is of the scene where the shy waitress decides to surprise her fiance by showing up at his apartment/hotel room, only to find out that he's just got married to his secret other girlfriend, who's pregnant with his baby. (I think the secret other girlfriend was a stripper - clearly this book was not big on the subtlety.) The shy girl loses it for a bit, but the main character and the party girl friend eventually manage to cheer her up through a heartwarming final scene where they all watch a fireworks display on the beach together.
Does this sound familiar to anyone? It's been nagging at me for a while, I'd love to just know the title.