''Ask her what to you can do to help.''
It's not that simple. A lot of people will deny that they need help and will say that they are fine when they are not. I just read a study about how depression can be caused by the social expectations by the western world to be happy. So they will feel pressure to show their depression and will try avoid asking for help due to the whole idea of being happy is right and being sad is wrong. But still, the least you can do is ask the person how you could help so don't end up doing nothing, at least try.