agree with their anger....but show them different ways of seeing the situation and the different solutions...and let them know that everyone makes mistakes and that person they are angry at might someday pull them from a burning building....a lot of times when we don't allow our eyes to open in love towards others to see what God sees....we will make the mistake of seeing the one we are angry at as the enemy...sometimes when so called enemies are stuck together in a small room...they are forced to see the other as human
but all in all if the other person is not saved and you are and they are seeking advice from you that is an open door to witnessing...if the other person is a Christian...remind them what God says about anger.
My teen daughter gets angry at her best friend a lot and I just say "oh you know you love her...you two will get through this...you will be friends again"
when you agree to the point of actually getting angry yourself...and you speak your mind about the person that made the person angry in the first place...and you verbalize your anger and say things that are not nice...it will bite you in the rear in the long run...it will because others like to get as many people on there side as possible and your words will get repeated...then if the person who was angry at said individual becomes friends again...then your words will get repeated so that it bonds their renewing friendship...
oh boy...and things get twisted.
you posted a scenario that everyone has gone through with so many different areas of life...