Concert or trip to the beach for a week??

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Okay so my question is, my daughter is almost 19 and her and her friends have planned a trip to the ocean for a week. Since that time a cyndi lauper concert plus some other singers are going to be at the same concert like blondie and some I don't really know of are coming. So my daughters fave is cyndi and now she doesn't want to go to the beach because of this concert. The problem is her friends and her have planned this for a long time and I know if she backs out she may not have any friends no longer. They will be really upset because they each have to pay $240 a piece to stay in a condo which I really don't understand why because one of the girl's parents owns the condo,don't know why they all have to pay and last summer they didn't, hmmm? It's 5 people going and here's the real kicker, the one friend who's parents own the condo is going to sneak and take her boyfriend and the parents have no idea, but I do, hehehe!! Anyway the boyfriend is the only boy going all the rest are girls. It really doesn't sit too well with me. So I want some good advice, I want her to just stay home and forget the beach because I see nothing but trouble coming but then again she's old enough to make her own decisions and I hate to say I would do this or that because then I'm the bad guy. I would like to tell the parents though about the boyfriend. Thanks for all your help! By the way my daughter and I have already seen Cyndi in concert like 5 times!!