How do I approach this in an adult manner

[Mosschops30]

Message withdrawn

mosschops make a formal complaint
your request was ignored after at least 2 verbal attempts
not on IMO

Definitely a letter, explaining how you has specifically requested on two occasions that that your daughter should not be placed with X. I would ask for an explanation as to how this occured and what steps will be taken to prevent such a blatent disregard of parental requests to happen again.
No need to say that nothing happened - that is irrelevant.

Message withdrawn

Can't remember how old your dd is Mosschops30 - but could she have asked to share with this girl?

If that is a possibility, it might be worth chacking with ther what happened before firing off a complaint letter.

FWIW - I think you should wirte a letter of complaint. Not sure how strongly you should word it - that depends on the anure of the group she was away with (eg volunatry/schol etc), but I think you should as a minimum epxress disppointment that your wishes were not respected, despite assraunces to the contrary, and what confidecne can you then have in the rest of their arrangements/any future trips.