I don't think it would be appropriate for her to give him her number right in front of you. However, maybe she could talk with you about him later to see if it might be OK. I'm speaking from experience. One of my good friends has a steady man, she's probably going to marry him. But she just meets these guys and develops these little crushes on them. (Maybe crush is not even the right word b/c she has no intention for a relationship, just harmless flirting). But I, and some of my other friends, am as single as they come. So if she meets a guy and wants to flirt and I, or another friend, think he's cute too we try to nip her flirt thing in the bud and just tell her another friend or I want to talk to him. We usually bring up the fact that she has a serious relationship and the rest of us are desperatly lonely and need a man

and she's like OK. It's really not a big deal with us and no one has come to blows over it.