Would you be happy about this?

[PileItUp]

If your ex moved into his girlfriend's house and then your kids had to sleep on the sofa every time they stayed with their dad?

Would you say anything?

YABU - it's absolutely fine.

YANBU - he shouldn't have moved somewhere they had no space of their own and you should raise it.

Is this an odd night here and there situation, in which case probably ok for now. Or a regular, weekly / fortnightly thing for more than one night?

YANBU this is not a suitable living arrangement. They need privacy, security, and an a decent mattress.

Unless he had good reason (eg financial problems) to move out of his flat, I would not feel happy about this at all. It’s not very welcoming and it could feel like their dad is trying to edge them out, especially to the teenager.