I have a 19 year old daughter, &, while she's technically an adult & can make her own decisions, she's still learning how to navigate her world &, sometimes, needs assistance and/or guidance.
In the OP's situation, I'd be very disappointed w/ my DD. And, honestly, I'd be hurt as well. I understand that grown kids have their own lives & want to spend time w/ their friends & boyfriends/girlfriends. A weekend w/ the boyfriend is going to naturally be more appealing than a weekend spent visiting family. I get it. I've been there.
However, that said, I want my children to know (& feel) that family is important too. If you wouldn't back out on your friends, don't back out on family.
After she was invited, she committed to go w/ her family on this particular weekend. Before the trip was planned, the DD was consulted to see which weekend would be better. Others (her dad & her brother) adjusted their work schedules to accommodate the weekend that would work better for the DD.
Going back on the original plans because "a better offer came along" is not kind & considerate & is NOT the adult thing to do.
And this is one of those instances where a 19 year old may need guidance in learning how to navigate the decision-making process & figuring out the correct, adult choice.
EDITED TO ADD - Our DD relies on 95% of our support too. So, no, I don't yet consider her an independent, self-supporting, free to do as she pleases adult. I don't hold our support over her head, but I think she does realize that we still have some say in her life.