Need legal advice for a credit card usage by ex boyfriend

Need legal advice for my cousin sister. She was dating a guy for over a year now and recently broke up because he was unwilling to speak to his family about her. Post breaking up he kept reaching out to her to rethink about getting back. She consistently said no. In the last week he reached out again and told her that he knows everything that she has been upto . He has access to her whatsapp messages , emails , contacts. We have checked and we don’t seem any linked devices anywhere unless her phone was hacked or cloned . He has now started calling her friends at odd hours sometimes even 4AM . He started harassing her to pick up his calls or else he will end his life. She has blocked him completely. We have notified his sister as well so she can speak to him

Now the difficult part is this , while they were dating, she had him issued an add on credit card which was not cancelled even post break up. He first trying making transactions on her credit card and when that did not work, he made a 2lakh transaction on the add on card. He is refusing to pay and says that while they were dating, he spent a lot of money on her and all of it needs to be settled. What sort of legal action can be taken ?

Her mistakes here were not cancelling the card in the first go, she has also deleted all his whatsapp messages out of annoyance before this kind of harassment started.