If your father is investing the money for the sake of your cousin, this is permissible and he must fear Allah while doing so. But if your father had taken this amount to invest for himself as if it was his own, this is totally prohibited and he must return the money immediately to your cousin.

Sheikh my cousin is orphan he is 14 years old. My father has his 16 lac rupees. So he promised him to return and he will return it in shaa Allah ones he(cousin) get mature mentally. But as for now he has used this 16 lac in business without telling him with the intention of returming him back. So is it permissible to use them and will there be zakah paid on this money.