It doesn't matter if your ex had his own status. You applied to sponsor him. This will come up in any invitation you send.
It's not at all clear to me that there is an error. You say that his extension was granted to December 2018, and that this was the date mentioned in the notes. What exactly do the notes say? Is there a chance this date is in the notes as the expiry date of the extension?
Then say that in your application. But be aware that you can say this and still have the right to sponsor him - you cannot write your way out of that right. And since you did attempt to sponsor someone else once, this is a risk in your profile.
To keep it simple, no where on any document/application indicated a stay or extension for 4 months. So their notes insinuate that his stay would be dec 2017 and he left in Feb 2018 which is incorrect. His extension was DEC 2018 and he exited Feb 2018. Hence this is an error.
In short, what I have to include is the application + copy of the extension to show he never over stayed.
Also, it is important to address the negative points from the GCMS notes. Since my previous sponsorship counts against us, can this be addressed this on the next application?