S.E.
I'd change up the plans and say something like - "you can go and I'll pay, but we'll go on this trip as a family and sight see and stuff on the way. And I want to talk to her parents before we go."
Then if it's a predator they'll probably cancel the plans.
If this girl is legit, she and her parents will contact you and help make arrangements for the trip.
I don't think there is anything wrong with him having a girlfriend in another town, but I would treat this girlfriend like I would anyother girlfriend my son had (and I have a 15 yr old son too). I have to meet her. I have to talk to her folks.