Can I tell my friend to drop someone who is making her depression worse?
Maybe. Why would you give her this advice?
Because she is the only person I like always there for me when I need someone
I am glad that you care enough to notice that this person is upsetting your friend, but are you sure that this is sound advice. It is not good to try to tell your friends who they should be around.
I just want the best for my friend and this is the best option she has to get better.
Good luck, but I hope you realize that it might not go well. Friends don't like to be told who they can hang out with. It might damage your friendship.