I think having a friend tell them is a lot easier on both parties (except for the friend, of course), but face-to-face has such a better pay off (for lack of a better word) and is definitely a lot less likely to be misunderstood. I vote face-to-face.(:
Face to face. They'll appreciate it and they'll know that if you actually liked them you had the courage to go up to them and tell them. You can do it! <3 xx
face to face! be brave and confiednt, trust me they'll appreciate it and be more flattered, but if your friend tells them they might think she's joking at first.