Don't tell her. Either she hasn't noticed and is therefore not bothered by it, and the problem lies with those looking at her. Or she has noticed and doesn't care - and again, the problem lies with onlookers. If she wanted to do something about it (bleach, wax, electrolysis, shave, laser, etc), she would. Believe me, I know. I have had one since my teens, and now have proper coarse hair on my chin and jaw too, which I now shave every day. I don't want everyone to see this, so I have tried electrolysis and laser treatment, neither of which worked, and am now shaving, which I hate, but I am fairly sure that most of the time, no one notices.
DS has commented on my chin being "prickly", and two kids have told me in the past that I have a moustache, and I found all of these comments mortifying. Your friend might feel similarly if you say something. I manage it as best I can, though can't be arsed to bleach my upper lip regularly (all this de-fuzzing takes time), and I am much better these days at thinking that any problem with it lies with the person looking at me. I hope your friend's self-esteem is robust enough to not give a flying f*ck about it either.