beckyvardy
It's different for everyone. For me, when it's bad I sometimes get intrusive thoughts of terrible things happening (the nature of anxiety means that the intrusive thoughts are the things you dread most). My intrusive thoughts are (currently) very vivid imaginings of awful things happening during labour/birth/awful things happening to my baby/DH. When I can't shake the thoughts I end up crying, usually. I'm lucky in that I have only very rarely in my life experienced panic attacks.
When I'm well I can quite easily identify that kind of thought as unlikely/unhelpful and distract myself or focus on more positive/realistic things. When I'm not well I can't, though. It really is like being stuck in a nightmare and unable to wake up.
It's not the same as depression but quite often people have both. If you have problems with anxiety then doing basic day-to-day things (e.g. going to the supermarket) can be very hard, so you end up doing less and less. After a while, your mood can start to dip. Often people who are struggling with anxiety blame themselves and tell themselves they are silly/stupid/useless etc and that feeds into depression too.
On the flip side, people who have depression tend to have very negative views of themselves, the world in general, and what the future holds*. If you believe that you are stupid and useless and the world is dangerous and everything is going to go wrong, you often end up worrying.
Best thing for your friend is to ask her what is going on, how she's feeling, what it's like. She might not want to talk about it, of course. If she does, just listen to her. Maybe empathise a bit "that sounds really tough" etc. Try to resist the urge to "fix" it for her or to try to reassure her out of the anxiety "But you're brilliant at your job! Of course they're not trying to get rid of you!" might be a completely natural thing to want to say, but that kind of thought needs to come from within her for her to believe it.
*There's a lot more to depression than this, but it's a common thread.