It's important for children to know that feelings are OK. It is also as important for them to learn what feelings mean, to put a name on that feeling. Have her identify pictures of people with various expression on their faces. What are they feeling? Why might they be feeling this? What can we do when we feel certain feelings and what can we do when we identify these feelings in others?
Learning to calm yourself down is an important skill for all of us, but if learned early in life, it can take a lot of stress of growing up and growing old. Teach her to Stop and Think. Have her ask herself how her body feels (are her jaws clenched, hands in a fist, ears burning) let her describe, let her talk her way through this. It gives her control and builds the skill. After acknowledging how she feels and how it looks, have her take deep breaths (pretend she is smelling a lovely bowl of soup, inhale and exhale), count backward slowly, think calming thoughts, or talk to herself. Any or all of these can be calming.
Once she is calm, have her figure out what it was that made her upset. How can she solve this problem? Will the solution work for her and the others involved? Is it safe? Is it fair? How will others feel about it? Will it work? If not, then brainstorm another solution until you and she come up with one that will work for everyone. This teaches compromise and problem solving.
She will feel much better knowing that she is having normal feelings and that she can figure out a way to get through them and problem solve.