Worried abt my best friend

So one of my realy close friends is going through some stuff right now. I really think she is depressed but she doesn't want to admit it. I have been through a lot if the same things and really want to help her but i dont how to approach it or talk to her about it. She keeps getting worse and following under bad influences and i dont want her going down the wrong path. Can someone please help me and give advice on what i can do to help. Thanks so much and i hope everyone has a great Thanksgiving.