I kind of think watching 'age inappropriate' films is the standard fare of sleepovers to some extent..
However, you know your kid and if they'd genuinely struggle with something its definitely good to give them an out like 'ill say you're not allowed, it's me thats the meanie' or 'I can call with an emergency and come get you' plan if necessary.
Does depend on what they're watching though... I wasn't easily scared by gore, I have watched all sorts of horror, Stephen Kings IT is one of my all time favourites, as is The Green Mile... and some very gory early 80s stuff, all the Hell Raisers, Elm Sts etc, all well under 15.. never mind 18.
I can't do jump scare films though, I've watched one of the Final Destination series and cannot watch the others! They have my adrenalin through the roof and can't calm down for days!
I don't like the more modern psychological wobbly camera stuff though, and I think it can lead to silly teenager hysterical behaviour in some kids.
Best bet is to have a chat with hosting mum, possibly mention that them watching 18's if they mention it in school, could stir up unnecessary trouble, see what she says.