Your coworker is wrong. The thermal camera measures temperature based on the amount of radiation (I) it senses:

I = s e T^4

Where T is temperature in Kelvin, s is a constant (5.67E-8), and e is emissivity, a value between 0 and 1. For most objects, emissivity is between 0.9 and 1, so the camera assumes an emissivity in that range and solves for T using that equation (T = (I/(s e))^1/4 ). However, metals have low emissivity, often in the range of 0.3 or lower!