She shouldn’t be and yet they together attack the carpenters home and seem to have forgotten all their differences blaming Harry and Michael.
This is exactly how evil puppets of the fallen would act, not how a mother whose child is just murdered would act. She did not ambush Nicodemus when he came out, she joined forces in hatred redirected.
A clear sign to me that the fallen had everything in control and had in their mind rewritten history already to blame Harry and Michael for everything and not Nicodemus.
"Tessa stood beside him in human form, wearing black trousers and a black shirt. Her expression was distant, haunted. She looked awful, thin and wasted away, like those movies of people rescued from concentration camps, but her eyes burned with some dark emotion that the word hate didn't begin to describe"
I don't know what's going on with her, but I seriously doubt it would take much to turn her against Nicodemus. A few carefully chosen words would do it I think. Something like:
"Your daughter looked so innocent, fragile when she handed the Coin to Nicodemus. She was just a child, and he murdered her. He didn't show any remorse. She was just a tool... Your little girl was property to him just like you were to your father when he traded you"
I'd imagine that would do it.