Did your friend ever teach her to down on cue? If not, then she's not "refusing", she just hasn't been trained yet. Have you tried luring her into a down with a treat?
One really easy way to train a down, which I do every time I bring home a new puppy, is to mark ("yes!"), and give a treat whenever the dog lays down on his/her own. Even if a dog doesn't know how to down on cue, they DO know how to lay down! If you toss the treat a few feet away she'll have to get up to get it. You can wait for her to lay down again, and then mark it and toss another treat. Do this for a few minutes several times a day. She will likely start offering downs in order to get the reward, and at that point you can use your verbal command right before her elbows and butt hit the floor. She will start to associate the cue with the act of laying down, and then you can start to use the cue before she's starting to lay down, and simply wait for her to do it. Mark and reward when she does.
Don't rush the process though, it may take a few days or a few weeks for her to fully learn what you expect.