I'm with Neon on this one. I'm of the mind that if you give your blessing to a partner's sexual intimacy with another person, than that is as far as disclosure needs to go. Unless they are involving other
people in that activity or there is a sexual safety incident.
His girlfriend has every right to complete privacy about what they do with their bodies in my opinion. If it is something you can come to a mutually comfortable compromise on though, that is great