Well firstly you can check Messenger's built in Message History. Open a conversation with the contact, then go to File > View Message History. If the option is greyed out or the old logs aren't there, you're out of luck and they haven't been saved.
As you have Plus!, this may have saved a chat log. From the contacts list, click the
button > Chat Log Viewer > Go to the date you wish and look for the contacts email address. You can also use the search function, enter your contacts email address to find all logs for them.
If you're unsuccessful with both of these, you're out of luck I'm afraid.
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