Did The Engineer Break My Machine?

New brushes tend to make a motor noisier at first, creating a ratchety sound until they've bedded in so that part is normal unless it was making more concerning noises which might have indicated excessive sparking, which should not have been left.

If the new brush was sparking excessively and he saw it but ignored it then he would be responsible but it's possible it seemed ok when they were first fitted but then started sparking heavily when you tried it with laundry inside. I always tested my repaired washing machines with a load of laundry to put a proper load onto the motor. Some faults can only show themselves under heavy load conditions.

Unfortunately it's not going to be possible to prove anything unless an independent engineer can say what he thinks happened. He could have done a bad job, fitted the brushes badly, fitted the wrong type brushes, or just ignored a clearly worn armature and hoped the brushes would bed in. But it could also just be that the brushes seem to be okay but the motor deteriorated rapidly.

If the engineer didn't seem trustworthy the only thing you could do is get a different engineer and hope he can work out what happened but if the engineer seemed okay it might not have been his fault. It's impossible to say without being able to examine the motor.