This is exactly why I support euthanasia for humans, too.
When my mother had COPD at end stage and terminal lung cancer, she was under intense pressure to quit smoking... all it did was drive her to smoke in secret, like a naughty teenager hiding behind the bike shed.
Eventually, I found out that the sources of said pressure were my step dad and my sister. I had a talk with my mother and asked her what the point of quitting now was??? I mean, she's terminally ill, the only question is what's going to kill her first??? Then I asked her what she wanted??? She'd been smoking from the age of 9, so 40+ years by that time. She wanted to continue - so I took my question to my step dad and my sister "What's the point in mum quitting now???". Of course, they had no answer, either. She was still terminally ill and her cancer was inoperable because they were scared that she wouldn't come round from the surgery. It was enough to get my step dad and my sister to drop the pressure and let my mother enjoy her remaining time however she chooses. If she quits, she quits, if she doesn't she doesn't... she doesn't need to carry on like a schoolgirl hiding from the Principal.
The same is true for your friend here... you might or might not agree with their choice, but, as long as they are mentally able to decide for themselves - it is their choice and all you can really do is support them in whatever they decide to do. At this point, there's no real point in standing against them because they'll likely do it anyway... the only variable is if they have support in whatever they are going to do or not.
