This is a tricky one isn't it?
I'm a super cautious parent by nature - overly so I'd say. My first inclination would be not to go tbh.
But then again, I'm torn.
Is it just you and he going on holiday? Kind of leaves you on your own worrying a bit doesn't it, with no one to share your concerns with?
How reliable is he usually? Good timekeeper? Comes home at agreed times usually/keeps to agreed curfews etc? You'd need to catch the ferry going back by a certain time no doubt? Would he be reliable to get back to you in good time? Confident enough to speak up and say he's had a great time but needs to go now? You'd need to consider all these things I think.
Have you seen her social media accounts? Are they visible? I'd screenshot the he'll out of everything as a backup.
(I did say I'm an overly cautious parent, absolutely no one come at me please).
Would he be open to using the Life360 app / air tag etc/snapchat (or all, as a safetynet purely as hes meeting a stranger in a foreign country), just in case he gets lost or something along those lines? If he's reluctant to that then that'd be a hard no from me.
Above everything else they'd need to be a grown up conversation about personal responsibility and respect to you and your concerns, whilst also respecting his feelings etc.
I do find this time of transitioning into young adulthood the trickiest to steer - they know it all don't they? So much confidence.