I have had a conversation with a friend private mess not on Facebook is there a way I can recover them if so please tell me thanks

If you can't find the messages, you might need to check out the archived files and see if they were saved in there: How to get back deleted messages on Facebook . If you can't recover them via this way, you have to rely on 3rd party software.

Check out the video tutorial below:

We use Facebook daily to do conversation with our friends, relatives and sometimes with our profession clients too.
It's very important for us to get those conversations back if mistakenly they can deleted.
Some common situations that lost Facebook messages you can recover from:
1. accidentally deleted it on your android phone
2. lost messages after Android 5.0 updated or other Android upgrade.
3. save messages from water damaged, crashed, broken or smashed Android phone
4. Android deleted messages randomly by itself
5. Factory restore Android phone.
And more...
Watch this video and share it with your friends too so that they can also get their precious lost data back.

Still worry about your deleted Facebook Messenger messages? Don't feel panic. You can follow this instruction to recover your important Messenger messages with ease.
http://www.jihosoft.com/android-recovery/recover-deleted-facebook-messenger-messages-on-android.html
How to Recover Deleted Facebook Messenger Messages on Android
Hope thia article can truly help you.

Check your messenger. They should be in there.