Not every tip you read will be appropriate for every person. People with anxiety are different than people dealing with depression, are different that people dealing with Bipolar are different than people dealing with a myriad of other issues, and even within all people with anxiety, they are all individuals. There is no one-size-fits-all advice here, other than to open the lines of communication, and ask how you can be a good friend.
In the end, isn’t that what being a good friend to anyone is? Trying to understand how you can be helpful and supportive in their life? It’s no different here.
But, these are a good place to start.