How should I tell my divorced Mom about my Dad's new baby?

I just found out that my Dad is expecting a new baby with his fiance. My parents have been divorced 7 years, but there is still a raging financial battle constantly being fought. My Dad refuses to follow their agreement and pay child support. My parents are in their late 40s and the baby is a graciously accepted "accident". I don't think my Mom has ever imagined that this would happen, because it is such an unusual cirstance. My Mom is going to find out about this eventually, how and when should I tell her?