She needs a reality check, any people I know who got married abroad booked it fully aware that not everyone would be able to go and accepted that and also stressed to invitees that there was no obligation to travel. Her family do love Lanzarote but they generally travel at the cheaper times of year not October mid term. Sometimes I was pricing flights to Lanzarote and you really would get to America cheaper.
I'd say a lot of people might be waiting to see if the flights go down before committing to going, so she won't be getting many rsvps yet.
I'm assuming they have their own flights booked, it wouldn't be the end of the world if it ended up just being the 4 of them surely, they'd still get some kind of a ceremony and nice photos in the sun as well as a holiday, it's just the attention and the cash gifts they'd miss out on.