Long read, but need advice.

Hello all. I am a manager and recently I had to deal with a disagreement between two employees ( this one can be A) which resulted in one of them abruptly quitting. The one that quit also sees one of my co-managers. The problem that occurred seemed to me to be miscommunication and misunderstanding between the two employees (we’ll call the other B) which led to A becoming unintentionally offended by B joking around. The three of us went discuss the matter privately during which time B tried to apologize and A was very hostile and dismissive. Ultimately I tried to politely explain to A that it seemed like the whole issue had been overinflated and that they had taken the joke a little too much to heart. This led to them storming out and abruptly quitting. How do I talk to my co-manager and explain that the situation had been overinflated and that A had been the aggressor? I genuinely enjoy working with this person and don’t want any bad blood between us at work, but I also don’t want to not tell the truth.

Side note, B has never had any other issues with anyone, not even A until this happened. They are always 100% themselves and always joke around with everyone. I work in an overall positive environment in which we are all comfortable and ourselves around each other and generally everyone gets along very well.