The DIY should have indicated applying anti-squeal to the rear of the brake pads and grease to the metal surfaces that come in contact with each other.
Brake squeal is generally caused by the vibration of brake pads. Anti-squeal provides a soft material that absorbes these vibrations as well as an adhesive that binds the pads to the brake piston.
Metal parts like the caliper dowels and the brake pad edges that mate with other metal parts should have grease applied to them to prevent wearing and ease of operation.
2004 330i ZHP ESS TS2