Air Profit & Nitpicker.
What would I have done?
I would have taken along a knapsack with plenty of water, high energy rations, a torch, spare shirt and trousers, a hat and a mobile phone. They work in most areas in the country parks and have been used many times to raise an alert. I would not have separated from my companion.
However, given the circumstances, when he did not appear at the bus station rendezvous, his mate should have reported to the local police station. He could then have led the police to the scene without delay. It must have taken him at least 90 minutes to get from the bus station to their hotel.
A simple Google search of "Hiking+Hong Kong" will reveal several sites with all the information you need to prepare for a hike in the hills.
I hope he is soon found safe and well.