Years ago, my best friend gently told me I was overplucking my eyebrows and it didn't look good. I appreciated the feedback and was more than happy to stop plucking so much.
I think you'd have to determine for yourself if you should tell your friend something like this, there are so many variables. Would she be able to take the comment in the spirit it's intended.... would you be able to put it in a way that won't hurt her feelings.... can you pick a day/time when she's in the right mood... etc.
Our poor husbands, lol. They have to walk this tightrope every day.