I'm going to tell my friend she's been overly harsh to someone who left a funeral service early
How was she overly harsh?
She had no right to be mad at the guy who had left the funeral service early, she had no regard of other's feelings.
Being honest with her is good. Being rude to someone like that makes no sense.
I don't care what her reasons are.
You should care what her reasons are so you can tell her why what she did was wrong. It is important you understand where she is coming from so she can understand your point better.