I'm by no means an expert on this but I can try to offer advice based on what would be nice to hear from friends of mine.
If your friend calls you during a panic attack it could help to remind her that it's her mind that's responding to something and that in 5-10 minutes (usual duration of an attack) she will be alright. You can also tell her that you'll stay on the line with her and be content to wait even if she can't talk; that you just want to know she's alright once the panic attack subsides. Tell her that taking nice steady breaths will make her feel better again.
If she calls to tell you about a panic attack she's had before you can tell her that's its a common experience and that anxiety is a very natural thing for people to have to deal with. You might also want to ask her what's giving her anxiety if she's willing to share, and whether she's taken any steps to treat it, come up with an action plan for her to receive help if she wants it (ie. reaching out to some therapists in the upcoming week), etc.
Hope that helps!