I understand what you are saying...and I'm not a parent, but to an extent I agree with you.
I do think, however, that her being 21...her parents should trust her enough, being a grown woman to make the right decisions...
I also think, that if you are "too strict" then all your children will do is rebel against you and end up lying all the time to do what they please. To have a good relationship with a child and ensure they are making the right decisions, you have to give them a chance (to prove themselves trustworthy)...especially at 21 years old!
It would be a different story if she was 16.
The question he asked was "what can I do to get her to go with me???" Obviously the parents aren't going to budge.