I wouldn't think this was appropriate and wouldn't be happy. Do you think this relationship will last? You say she is a new gf. What if in a few months they split up, he gets a new one and she is sharing with another child. There needs to be some boundaries and sleeping in her own bed would be an unnegotiable one for me.
My children did the first sleepover at my exs new gfs on sat (only met her 3 times so i wasnt happy but i always try to be reasonable) however they came home yesterday and told me that my dd7 shared a bed with her ds6. I told him i didnt really think it appropriate especially as there are enough beds for it not to happen and he thinks im being silly. I know they are innocent children but i think they should be protecting them from day one i know full well one badly timed comment could cause a whole heap of problems. Im not happy about them sleeping there but hes planning on moving in with her asap so i sucked it up because they need to get to know her and be happy as when hes living there they will spend every other weekend there, but i just want my one wish listened to.
Ready to accept im being silly over it, and if not where do i go from here to make him listen.