CRAP THIS IS URGENT HELP:( PLEASE!?

Ok so me and my friend hacked this mean girls account and started talking wierd stuff to her crush but told him it was us. So now i deleted the chat thing and message thing but whenever he messages her or she messages him she can read all the messages and will be able to tell it was us:( so I was wondering if there is any possible way to delete them FOREVER?!?! plese help . Shanks Yew:)