What would you do - when feedback isn't heard

A member of staff has been spoken to by their manager about concerns with their performance and professionalism. They agree to make improvements and leave the meeting appearing to understand what is expected.
However, afterwards they begin complaining to colleagues about the manager, saying they were “picked on” and that management “doesn’t know what they’re talking about.” They continue to criticise the manager to others and show little evidence that they have reflected on or changed their behaviour.
colleagues feel uncomfortable about being drawn into the negativity.
WWYD?