Hi, I'm Carl and welcome to Just Answer. In Plenty of Fish, I know before you could goto send a message then in the To: ***** ***** it to the name of the contact you deleted messages from and send a test message. From there is would show you the hidden messages you had with that person from the last 30 days. It may have changed now. If it did, the only way to retrieve them is to request a restore on the account. The final method would require a subpoena if this were required for court.
If you need any clarification or additional help, please feel free to ask me.