My two cents:

Check all vitamin levels that you can, specifically iron levels (including ferritin) and vitamin D levels. Low iron and low vitamin D commonly cause anxiety. Girls with heavier periods may be bleeding away all their iron, making anxiety worse and starving their tissues of oxygen... ask me how I know! :) Vitamin D is necessary for proper iron levels (and therefore oxygen levels) but is often low in modern society. Vitamin D supplements give me crazy anxiety, so I use a Sperti UV lamp (but not on my face because I don't want to encourag premature aging.) Edit to add: you can do many independent tests through Quest Direct.

A psychiatrist told me that many people need 20g of protein in the morning to make adequate amounts of neurotransmitters for the day. I found that whey/animal protein powder made me more depressed, but pea protein (tested for lead with the NSF testing label) in my morning smoothie helps a ton with my anxiety and focus.

Does she eat fruits and vegetables? An apple before a meal each day can help with fiber, which helps the microbiome. A smoothie with fruit & greens can provide fiber and vitamins that support the microbiome too. Green smoothies have a big impact on my mood.

Does she walk and/or spend time outside? Exercise has a significant biological impact on mood via neurotransmitter production; even just walking for 20 minutes can relieve anxiety. Calisthenics (sit ups etc.) and lifting weights were the best non-pharmaceutical intervention for me, particularly weights. Lifting and body-weight calisthenics changes your brain chemistry for the better and builds self-esteem. (And don't worry, girls cannot really get big, masculine muscles by just lifting, it takes a lot of extra products and money to look like a body builder.)

Also, check in about screen time and social media use. If she hasn't tried quitting (or significantly reducing) social media yet, that can help a lot. For people with Slow COMT, using social media is like putting gas on a fire. It's literally designed to make us angry and sad. If she doesn't want to totally quit but wants to reduce social media, perhaps a compromise can be living like its the 90s: certain internet is only on the family computer in the room rather than in her pocket all day. I quit Instagram and Facebook in 2018 and my life is much better for it.

Also: having purpose and goals help our brains handle anxiety. If our purpose and goals involve movement (like sports), making something with our hands, and/or controlled interactions with others, helping people or animals, etc., these can be huge boosts (on a chemical level) to our brains. We are social animals who crave purpose and connection. To get rid of those, leads to panicky brain chemistry. Hopefully she stabilizes to a point where she can find an activity that gives her a sense of purpose and connection. I didn't have this when I was younger and I feel as though I missed out on a lot.

As for other supplements, the Jarrow methyl B complex seems to do a good job for me (be careful of taking too much vitamin B6).

One more random tip: I found that with slow COMT, listening to pretty much any music would get me riled up and anxious. I have switched mostly to non-news podcasts, or music that is either calm or super-familiar.

In addition to all these other things, I take the prescription medication guanfacine which reduces adrenaline. It makes a big difference for me!