Defending a member in a trial/adversarial process and a cooperative relationship do not exclude each other. The union can very well defend the member in this case while still discussing cooperative solutions to the general problem of officers bullying citizens, just as a lawyer can speak for your side while still reaching out to the other side to find a solution that's acceptable for both sides and does not include a verdict.

It is however important that unions have a bargaining power that is at least comparable to the other side's power - and that sometime requires going head to head with the opponent.