The child has no locus standi to file a case against the lady. If his parents are divorced the dad can meet any lady legally. If the lady is married then her husband can file a suit against the dad.
Stealing affections of dad is not a crime, because if he is divorced, he has a right to marry again or otherwise lead a social life. I am not sure there is any scope for any child welfare organizations in this case, unless the lady is mistreating the child.
You have not been clear who the property belongs to. If the property belongs to the dad and the child is only the heir, he has no right to restrict anyone from entering the premises. If the property belongs to the child and the father is the caretaker till the child attains majority, then the child will have to file a suit for change of guardian citing the inadequacy of his dad to be his guardian and caretaker of his property. Then only can he file a suit for trespassing against the lady.
In any case, who are you and what is your relationship to the child. If you are the child, then I would suggest that you put more efforts in your studies and your future rather than trying to curtail the social life of your father. If it is some 'well-wisher' of the child, then please stop poisoning his mind against his father.