What if the team members of the certain manager are not productive enough and management do not listens to the manager.?

I faced a situation where my friend was a team lead with a team of less qualified and average qualified personalities. The most difficult process was to make the team make understand things as required and then generating the output.

The process which he followed:

1. Trained the members for 6 months as per the work and company requirements

2. Worked on their mistakes and provided an output for another 6 months (totaling to 1 year where members got habitual to the work process)

3. After 1 year, the team lead (who was now manager)
stopped working on the teams mistakes and tried to persuade them to omit mistakes and provide a good output.

4. Now here, the team went against the manager and provided lazy attitude and demonstrated poor efficiency and working skills where the manager had to say them argumentive words so that, that individual feels it in mind and performs well. But all went against him, where slight scoldings became severe due to no response from the team.

At the untolerable stage, 3 members of the team resigned on the same day. Now here management awoked and consulted the team members and conforted the manager for his wrong deeds which lead to such a situation, without hearing what actually happened and what were the reasons for this.

With this, manager resigned as he could not find solutions to his problems.

Now in such situations, can we consider an employee a MEAN or the mistake lies somewhere else?????