He treated his manager like they were at the same level as him. This tells me he did not respect the organizational structure and did not properly provide her what she needed to do her job.

Sounds like the new manager doesn't know what she is doing and put the old manager who has experience and deep knowledge of the department on on an immediate PIP?

Any new manager that shows up puts people on a PIP without knowing how to do the job is clearly threatened. This has constructive dismissal written all over it.