I am with the OP, this woman's message suggests she has no idea of the enormity of what her son did.
Yes, it may not be her fault he behaved as he did. I can accept also that what he did was when he was a young teenager and perhaps confused or even mentally ill at the time and he now deeply regrets it.
BUT
whatever the reasons for the offence, surely anyone with even a small fingernails worth of empathy and understanding in their body would know that after an event like that, those affected by the crime are highly unlikely ever to want to see or hear of this boy or his family ever again. Most of all one expects the boy and his family would understand this.
This woman's message to the OP and daughter is crass and ignorant and possibly goes someway to explain the boys behaviour. He lives in a household with no proper understanding of the boundaries that should exist between people on social and sexual levels.