I do agree that she should know, the part I disagree with is you being the one to tell her because it sounds like that will just cause more trouble. It should come from your dad, but if your mum is the type that won't allow mental health care then there is no compromise or agreement to be made there, that's just her straight up being a neglectful, abusive parent (which is why you'd be better encouraging him to take it through the court so he knows 100% where he stands legally).

It just boils down to what the court would find more serious: a parent ignoring and/or denying necessary medical care to their child, or a parent not informing their co-parent that they were seeking necessary medical care to their child because their co-parent wouldn't listen and had a history of medical neglect. Keeping in mind, the priority for the judge is doing what's best for the child, the better parent is best, the court order can iron out some of the issues with co-parenting. I'm assuming if you got criticized in court, that would be for bad co-parenting, not bad (abusive) parenting.