Life in constant pain, pratically immortal, death is the only relief. What to do?

We're reaching a time where the immortality of an individual is almost possible, and in case this happens, a moral dilemma resurfaces: if a person's existence only brings him pain due to a constant condition or sickness that can't be cured and is kept in a way where he can't die and death would be their only relief, is it moral to end their life out of misery?

There was something similar that happened in Japan in 1999, where a man who suffered a boatload of radiation began to decompose while still alive, his flesh collapsing onto itself in a horrible manner that brought him pain and suffering. At that time, the most moral decision would've been to relieve that man of his pain, but the scientists were fascinated with him and kept him alive for 83 days with all the medical equipment they had. He suffered constant pain all the while and pleaded with them to kill him until he could speak no more.

Although their intention was not limited to keeping him alive but also to studying him, this behavior isn't any different from what Christians expect people to do to their loved ones on death beds. I'd argue that the most humane thing to do sometimes is take the pain out of a person and let them pass on. Some of us have been doomed with conditions that make living a borderline torture. I'm aware that some people were able to live nonetheless and inspire others to do the same, but not all of us are the same.

There was a recording of a woman, which sadly i've never heard or seen so i cannot give source, that had suffered chronic depression for all her life, and during a brain surgery they did to help her live a normal life, she suddenly said something along the lines of "Yes! There! There's no pain—no, no, no, not again...not again!" She had, for the first time in her life, felt free from her chronic depression, but due to a sudden movement from the surgeon, she had to re-experience the pain she had felt for her entire life. It's unclear what happened afterwards; i could not find anything about it. In that case, what is the moral thing for this woman to do? Live on through the pain?

I'd argue that for some people, a privileged life in which happiness can be experienced if they just shift their point of view is not possible. We are the lucky ones. I don't think it's moral, ethical, or right to impose our standards of living on them.

Let me know what you think.