Not this.
The parents seem to be treating son as an adult, and therefore wouldn't even consider consulting parents of the partner.
His behaviour is the concern, OP, from your drip feed. Stop focusing on the parents, ignore people who think that inviting someone on holiday makes you a sex trafficker, and guide your daughter through this unhealthy first relationship. Make sure she knows the risks and the concerns, and guide her to make her choice.
Oh, thanks ever so much! Equally, to this^, I'd say, OP, not this!
You can't control him, or his behaviour. You can look out for your daughter's best interests, as you are doing.
These weirdos are not inviting the young girl to a weekend in Bognor Regis. I find it all very odd. The insistence she must come, especially. During a global pandemic, where flights are cancelled and quarantines declared, and the whole thing is a moving dumpster fire.