It is not advisable to place a hot water bag right away. Which doctor did he consult? Did he not ask that doctor? Please consult an Orthopedic specialist since it is not just some general pain but Dorsal Spondylosis so if you find somebody who specializes in the particular area of Spondylosis, even better. The doctor will give the correct advice on whether to use a hot water bag or to do a cold application first or some other kind of treatment. It is also essential to ask the doctor how long the application is to be done in terms of minutes, the number of times in a day, and for how many days.
The reason why haste should not be made in applying a hot water bag is that heat application may reduce the pain, but there is the possibility of the heat causing swelling and inflammation. See, there are so many aspects to pain - internal damage, external injury, swelling in the area, the extent of the pain (whether confined to that area or radiating to nearby areas), etc. I am advising this on the basis of my own experience. Some years ago I had pain in my ankle due to a fall. I had the sense to consult an Ortho immediately and he said to soak the foot in warm water for a short while and had given medicines to control the inflammation. If I had not consulted him, my first reaction would have been to put on an ice pack and expect the pain to just go away, which, in fact, he said not to do. He said swelling and pain are not necessarily controlled with an ice pack and the most imp. aspect is to control the inflammation that may creep up unknowingly. A follow-up is equally essential for the doctor to advise further.
So your friend need not take painkillers if he does not wish to and can say so to the doctor, but if he is ok with medications given for controlling inflammation, maybe that would help him. Naturally, it goes without saying that he needs to inform the doctor of any other health issues, however minor, and inquire if the medications given have any side effects (such as increased acidity, loose motions, etc).
Also, has your friend done a blood test to check D3 and B12? Sometimes lack of Vitamins can cause pain and he may need the necessary supplements (tablets) for it. I had severe back pain once for no particular reason and it turned out that my D3 levels were abysmally low.
Finally, I reiterate the importance of consulting a specialist to get the right advice.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell