Its odd that he would quit before his pip. I know several people who intentionally go on a pip if they have another job opportunity. A PIP is like, you can quit now and stay gone for 5 years and we will give you a big chunk of change, or you can stay on and fight to keep your job or stay at the company.

People who quit to avoid a pip are either expecting to work at the company again soon, or are total d-bags at work and worried that it will mess up their next interview.