How would you handle this situation?

One of my best friends is in a tough situation and I don’t know how to offer advice/ help them deal with it.

There are three people in this situation, I won’t say names so I am going to refer to them as Person A, Person B, and Person C
Person A and B are best friends. Have been for a while.
Person B and C used to date in middle school for two years. Now, all three are in high school.
Person A and C are now dating, and Person B doesn’t like really like that because it’s not very nice to date your best friends ex.
Person B has tried to ask Person C out as they are dating Person A.
Person A got upset and is no longer friends with Person B.
Person A and C are on break right now so that A and B can work things out. Person A is very upset about this because they are worried about losing two important people in their life

I really want to help them because they are my friend and I hate to see people hurting.

I hope I explained things in a way that made sense, I didn’t want to use names. If you have any questions, please ask.