Just for some background: I've been with my fiancé for over 2 years and he's been faithful the entire time. He has a child with a woman who once was his high school sweetheart and they have a amicable relationship. Once in a while they'll talk on the phone and she'll ask him for advice. Not too long ago, she was suicidal and he was the one that "saved" her and prevented her suicide by trying to get her admitted to a suicide ward and letting her know that she wasn't alone. She sees him as the one who saved her life.
Now to my issue. My fiancé has a high school reunion coming up and it's a 12-hour drive away (he hates flying and loves road trips). I can't go because I've already requested enough time off of work due to the wedding and events related to the wedding. Last night he told me that he was talking to his ex-girlfriend/son's mom on the phone and they were getting up-to-date on each other's lives. He told me that, while they were talking, they got on the subject of my fiancé's best friend, who has cheated on his wife numerous times. She told my fiancé that she had almost hooked up with him in the bathroom at my fiancé's first wedding reception and would hook up with him while they were at their high-school reunion if he wanted to. Later in their conversation, she asked if she and her daughter could ride to the high-school reunion with my fiancé in order to save money.
I have an issue with this because she's already established that she has no issue messing with someone's relationship. She sees my fiancé as her savior and they have a background. I know we need to save money and it would be easier if my fiancé had someone to split the gas money with but I don't know if it's worth the worry. I'm curious to know other people's opinion on this. I trust that he won't do anything but I don't trust her not to try something.