If her reason is that "she really has her heart set on trying out this new place, and can't afford to invite more guests there", I think the better solution would be to have the larger group at the more affordable place, and try out the new place with her boyfriend alone, as a couple.
Who wants to "try out this new, rather expensive place" to treat a group of people, anyway? There would be too much risk involved for me! What if the place turns out a disappointment? Then she is out of all that money to treat her friends. If I wanted to try out a new place, I'd go alone or with one other person first, to make sure it's as good as I think!
But if she /really/ wants to make it a "girls-only" thing, then I think it's really weird and uncomfortable to include her boyfriend. He is not a girl! I personally don't "get" or "do" girls-only things, but if I did, I would know well enough not to include one man. The heck??? That's just bizarre.