To continue @tom_g‘s theme, there is this quote from Buddha: ”Holding on to anger is like grasping a hot coal with the intent of throwing it at someone else; you are the one who gets burned.”
As @rooeytoo explained, you can only control your own words and actions, you can not control anything or anyone else in the universe. My son has had anger management problems. When he was a minor, I had legal responsibility for him, but I still could not control his actions in any way. When he was a teenager, there were times he frightened me, and I had to call the police on him. He has been through therapy and figured out his own ways for coping, but I am still concerned that there are unresolved issues beneath the surface. Those problems are his, and even though mistakes I made in the past may have contributed to them, they are completely out of my control. All I can do is be available to him for emotional support and to offer advice when he asks for it. If I nag or lecture, he will tune me out and avoid me, and eventually develop resentments.
So all you can do for the person you care about is talk to her – ideally, when the moment has passed – tell her how you felt in the moment of the incident, and that you are concerned that she might create more problems for herself if she lets her emotions control her behaviors. Share the quote I posted above, or perhaps another you might find. Then let her know that you will be there to support and encourage her if she chooses to seek help. Then leave it alone. Being honest is the first step, expressing what you’ve witnessed and why it concerns you. Then placing ownership of the issue in her lap, and offering support – without sounding judgmental or telling her what she should or shouldn’t do. If another incident occurs, ask her after things have calmed down, if she’d thought about what you’d said, and would she like to discuss it further? Be prepared to hear and accept “No” as her response, because even if you didn’t use judgmental phrasing, there will likely be some embarrassment on her part. Do not push the issue any further, because again you risk turning embarrassment into shame, and may wind up alienating her. If there is another incident, and especially if it happens in your presence, you can explain that you don’t like being around her when she gets that way, and limit your interaction with her.
As @rooeytoo noted, Al-Anon is an excellent resource for materials explaining why and how to detach yourself from a loved one’s issues. The material they have focuses on alcohol abuse, but can easily translate to other problems.