Hello from JustAnswer.
Switching to a different opiate may help, but it usually does not help in this situation. You are correct that 2 of the 30 mg morphine has about the same potency for relief of pain as the four 4 mg Dilaudids. However, when switching to an equivalent dose of a second opiate, you also will usually get an equivalent amount of effect on the respiratory system. All opiates can suppress breathing, and it is a direct effect of action on the opiate receptor in the breathing center of the brain. Contrary to many side effects, such as nausea and vomiting, that may be eased by simply changing to a different opiate, such as nausea and vomiting, the effect on breathing usually is not significantly different when changing to an equivalent dose of a second opiate. There is no problem with trying a different opiate, but there is a good chance that it will not help in this situation.
To some extent, when addressing pain management in someone with cancer pain, relief of pain is a higher priority than easing side effects. But if the side effects is more bothersome to him, then it is appropriate to consider different treatment, but for relief of the respiratory side effect of opiates, it would usually need to change in regimen to a lower dose, or the equivalent of a lower dose of the opiate. If his pain would become significant on a lower dose of the opiate, then it may help to consider the concurrent use of a non-opiate to augment the effect of the lower dose or perhaps interventional pain management procedures. Whether an interventional pain management procedure would be an option would depend upon the nature of the pain. For example, someone with a greater amount of pain in a limited location from metastases may get some relief of symptoms with a nerve block to that area. Even if there is pain elsewhere, if the nerve block can ease a site of greater pain, then the lower dose of the opiate may be able to control the pain elsewhere.
Regardless of whether a different opiate is tried or another medicine or intervention is considered to augment the effect of a lower dose of the opiate, the first step would be a discussion of pain management and the side effects with his doctor, including a discussion of which options may be helpful to him. As a friend, you may not be accompanying him to the office, but either your friend or anyone that accompanies him can bring up the topic with the doctor.
If I can provide any additional information, please let me know.