Alex Underhill was walking with a friend at the Jebel Jais mountain in the Ras Al Khaimah region when he fell and hit his head.
The 25-year-old, who was originally from Hampshire but lived in the UAE, had started to feel faint while out in temperatures that peaked at 45C (113F).
Mr Underhill, a senior manager for the Al Khayat Investment Group, died before rescuers could reach him.
George Crewe, who was hiking with Mr Underhill, described the events as the “worst day of my entire life”.
He wrote on Facebook: “This guy was not only my best friend but he was like my brother, my right arm, my right leg… we literally did everything together.
“We both got to a certain height and made the decision to turn back as it was so hot and we didn’t think we would make it on the water we had left. On the way back down we both felt very faint and exhausted.
Mr Crewe, who lives in Dubai, said he was walking ahead of his friend to try and find shaded spots where they could cool down.
“After a while I could not hear him so I turned back and could not see him,” he wrote.
“I went back to search for him to find that it looked like he had slipped and hit his head and he was unconscious.”
Mr Crewe said he did all he could to help his friend, but was unable to call for a rescue helicopter because neither of them had a phone signal.
He said he ran to get help and called for a helicopter, and “went with them to find him but before we got there it was too late”.
The Foreign and Commonwealth Office says it is providing support to Mr Underhill’s family following his death on 20 June.