How should I go about discussing money with a friend?

A few weeks ago my friend asked me to take some photos of her. I am a photographer (with both a college education and professional equipment) and she is in need of an updated head shot. She was pretty vague about what she needed so I asked her to get back to me with the details. Today she emailed me in hopes of scheduling a shoot. She has not mentioned money in any conversation about the photos and I’m unsure how to broach the subject. If she is commissioning me for professional work, I feel I should be compensated. But given that she’s a long time friend I don’t know how to say, “what are you going to pay me?” Any suggestions??