It is utter nonsense to strip this mother of responsibility for her loveless letter by blaming her religion. She's no more brainwashed than the next person.
She needs to be held personally accountable for choosing to not only shun her son but formalize it in a letter.
The world is full of people who say, "I had no choice...I had to do it". Rubbish, mommy dearest.
My mother was an active Witness and quietly chose not to shun me - the elders put pressure on her and she literally flew into a rage telling them to mind their own business.
They never talked to her about it again - and she remained in good standing in the organization.