Ask a female friend to hang out, but what if she has a boyfriend?

I've known this fem friend since elementary school. I used to like her back then, but I just see her as a friend now. I wanna ask her if she wants to hang out some day to "catch up on things", but she has a boyfriend, and I'm not sure if it's OK to ask her now. Should I just be clear that us two hanging out will be "strictly friendly"? Either way, I am interested in another girl right now, so I really wouldn't hit on my old friend.