If you notice it was a family "spat" and it was family members that were attempting to sue him. Monty has said some bad things about his father and has pictures in his book to prove it. Of course that will make the family angry.
It is not unusual for family members to turn a blind eye to abuse within the family. There could be anger and jealousy as well that Monty has been so successful (and rich). If his family is horrible, I doubt he will give much of that money to them.
It is also not unusual for an abuser to target just one child. Maybe his dad was "nice" towards others. I had a friend at school who's step- mom was absolutely 2 faced. She could be so nice towards strangers and other adults, but the way she cursed and belittled my friend was just horrible. My friend ended up running away twice and finally moved in with her cousin's family.
I don't think any trainer is perfect, and accidents happen. People can get hurt even with a well trained horse. Lawsuits happen especially when you are rich!