My bestfriends friend cuts and she hasnt been eating she hasn’t said an ything to us but we see it I’m not really close to her and and best f is bad with words so we not sure how to mention it ?

I think the most important thing is to not make her feel pressured and rather make an offer. You could for example say you are always there if she wants to talk about anything, no matter what it is and promise you won't judge her. But it can also be good to give her the feeling she doesn't need to speak about things in order to receive support. You could tell her she is welcome to let you know if she needs anything from your side and make a range of offers like talking, doing something nice as distraction, supporting her with daily life taks etc. You could tell her you are aware it can be hard to talk about difficult topics and tell here about options like 7cupsoftea where she can vent anonymously without having to speak. But it is also important to accept if she doesn't want support as for now. You can't force her. It can already make a difference to let her know that she is seen and that you do care and want the best for her.

Maybe you three could get together and hang out so the three of you could get to know each other more, then you all can sit down and tell her that you care for her and that you are concerned about her well-being

I had the same experience but was the one who struggled with self harm and eating problems. If you are a minor, I would talk to her parents about what you've noticed because they can help her the best. If you want to speak directly with her, just make sure she knows you are not ashamed of her and you care about her. She may become defensive but letting her know you are there when she's ready for help is sometimes the best and only thing you can do.