He says he fell asleep with a heating pad on his arm and this is what he woke to. I'm not sure why he was sleeping with a heating pad in the first place. The arm looks bruised, and that would not be a typical color for a burn. I'm wondering if he was sleeping with a heating pad on his arm because it was bruised.
Usually, bruising is treated with cold rather than heat. Some docs say you can use either, but the rule of thumb for most healthcare providers is to use cold to decrease swelling and bruising shortly after an injury (RICE). Heat can be used in a day or two to promote healing.
Either way there's only one solid rule: don't leave ice or heat sitting on the injury too long. With too much time, heat may cause a burn like this one. Ice, on the other hand, can cause frostbite if left on an injury too long.
Any opinions expressed here are for educational purposes only and are not intended for diagnosis.