How did your friend tell you it bothered her? I have misophonia and I can't ever come up with a polite way of saying "IF YOU DON'T STOP MAKING THAT NOISE I'M GOING TO DISEMBOWEL YOU THROUGH YOUR NOSE!!!!"
I recently purchased ear plugs so I can put one in my ear closest to the person making the noise, and it works fine right now because it's winter and I keep my hair down for added warmth, but in the summer I keep my hair up, and the ear plug is going to be much more obvious and I don't want to hurt her feelings. I know I'm the one with the problem.
Edit: For those of you who don't understand misophonia, this is literally how I feel sometimes.
http://www.thefarcereport.com/loud-chewing-drive-you-crazy-how-to-stop-this-pet-peeve/