I have found calming essential oils really useful to help calm anxiety. Lavender, Ylang ylang, Jasmine are all great. If she is too upset to use them herself you can always apply them to you or get a diffuser and then discreetly be around her. Sneaky but works.
A similar approach is meditation - free and does wonders for the chronic anxiety I inherited from growing up with my BPDmom. Again, it is best if she can be persuaded to do them herself, but if she does not want to co-operate there are many lovely tracks on Youtube. Find one you think is soothing and play it (very quietly) when she starts to dysregulate. Here's one of my favourites:
https://www.youtube.com/watch?v=OBgN849_nOs&t=159s My adrenaline levels tend to go into overdrive when my partner dysregulates. Unfortunately if a couple is close these things can cross over faster than we think. So I have found it important to be calm myself at these times, deep breathing and thinking of butterflies, flowers and beloved on a good day

Seriously though, mood stabilization is important because if we are calm at least we don't make matters worse.
Hope these simple tricks are of use!