Chileans entering on ESTA tend to have zero questions asked because low risk country and the CBP officer assumes they don’t speak English, and they tend to arrive on flights that have people from high risk countries so CBP has better things to do with their time.
If asked why she’s here she can honestly answer “vacation”, since that is true, and the officer won’t ask a follow-up. She is not required to volunteer that one of the activities she will be doing during her vacation is getting married.
All the other advice here is true about showing intent to return, etc. if marriage comes up, but given the fact pattern I would be shocked if any question was asked at all.
My advice would be to have all the paperwork needed to show connection to Chile, but to be surprised if the CBP officer speaks more than once during the entire interaction.