The brief
The B180 had a check brake warning on the dash, which on a Mercedes usually means the pad wear sensor has been ground through and the front pads are at the end. He brought it in, the right call, that warning comes before the grind. The pads are the wear item, designed to be used up and replaced, and the discs they clamp wear too, thinning and scoring over their life. When the pads go right down the sensor warns, and the wear runs into the disc surface, scoring it. By that point you're not just changing pads, the discs are scored past serviceable, so the front needs pads and discs together.
The diagnosis
On the lift the front brakes confirmed it: the pads worn near the end with the sensor ground through, and the discs scored and below minimum thickness. The calipers and slides were freed off and checked, they were fine, just the pads and discs gone. The rears still had life. So it was a front axle set: new genuine Mercedes-spec pads and discs on both sides with a fresh wear sensor, because you don't fit fresh pads to a scored, undersized disc, and you do the pair so both sides bite the same.
The work
Both front discs and the worn pads were removed, the hubs cleaned up, and a new genuine Mercedes-spec set of discs and pads fitted with a new wear sensor, the calipers and slide pins cleaned and greased, every fastener torqued to spec. The brake warning was reset and the pads bedded in so they'd grip evenly from the start. A road test confirmed quiet, even, progressive braking with a firm pedal, the warning light off.
The outcome
Quiet brakes, a firm pedal, even bite, sharp progressive stopping, and the warning gone. The B180 went home stopping properly again. Worn pads turn into scored discs the longer they're left, so doing the front pads, discs and sensor as an axle set put the braking back where it should be, the safety job you don't put off.