He should come down and meet her in person for a day trip at least once before they commit to any overnights. I'm quite surprised he hasn't done so already, in 6 months.
Putting aside the dangers involved in arranging a weekend at a stranger's house, I'm an adult and have taken a chance on a weekend staying with with 2 men: 1)very new but had dated and knew a bit previously and 2) who i hadn't met but we got on over the phone for a few weeks.
Both were spontaneous with low emotional investment plus I had the money to get a hotel if stuck somewhere for a night and the life experience to say ' I'd prefer to leave now, all the best' and go.
Both times nothing risky happened, no 2 was who he said he was etc. However, the romantic spark didn't endure and tbh the whole thing was a bit awkward both times. No 1 was actually quite rude to me at one point and I slept in the spare room.
No harm done but she's a teenager who has invested 6 months of her time and emotions so could be very disappointed and also less well equipped than someone my age to manage the situation if she was not comfortable and didn't want to continue the visit. As a 30- something taking a chance, it was water off a duck's arse but I would have been very uncomfortable and disappointed as a 17 year old with high hopes for love.
I'm sure they will say things like 'even if we only hit it off as friends in person, we can still have a great time' but she will be the one either stuck in his home or having to let him and his family know she wants to leave.
In short, I think a weekend is far too much pressure and risk for a first date.