Assuming it's siliconed on around the edges, a sharp blade is best. Cut right through the silicone joint.
If it's tacked in the middle as well, then you won't be able to get at it with a silicone eater. After cutting the edges, it's probably just a matter of prising all around (uniformly) until the silicone comes away from the wall.
Using small timber or plastic wedges spread evenly around should keep the pressure even to avoid shattering the glass.
If, as the Builder says it has spots of silicone behind the mirror, what you need is an old cheese cutter. Failing that a strong thin wire. Attach it to two bits of wood and try to pull it from side to side behind the mirror cutting though the silicone as you go.