What to say?

That will depend on the situation and the person. So if your know the person, you will know what works. Like for me, I prefer to talk my heart out when I am upset. My husband is other way around. So I have to give him space and time to calm down. Later I serve him his favorite food when he is clam and then he pours his heart out.

First remind them to take deep breaths. After they are fully calm maybe talk about positive things to remove the negative thoughts. Easier said than done but first if they are angry they should breathe first.

If she is in really bad mood better stay away as her mind is still not ready to accept comforting from a friend. Let her steam out at first then when you see her calm already that is the time to come in and comfort her.

@Starkinds The only thing I can think of is simply saying something like "I can see you're in a bad mood. Please just know I am here to listen if you need to talk." And then just walk away. Usually when people are in a bad mood, they want and need to be left alone. If they wish to talk, then can work it out together on what to do.

If someone is in bad mood, we need to know what gets back then to good mood...based on that we can tell what to say them. However, doing or talking something related to their hobby may uplift them.

That is natural. A natural reaction of a person who is in distress. It will die out naturally. Just don't irritate that person.

I will just step away.