Your concerns seem justified. Have you expressed them to your friend? It's possible that lower nicotine makes the juice taste better to him.
It's also possible that he thinks he's not really quitting if he still consumes nicotine. It's a common mistake to think that way. If that's the case, maybe you could just inform him that nicotine is not a carcinogen and that it has benefits for many people.
Yes I think you got it right. I have already tried to express those concerns to my friend, but I don't want to sound too much like I'm preaching.
But thank you very much for your suggestion : I had not considered too much the nicotine aspect, and the fact that my friend might be thinking that he is not really quitting because of his consumption of nicotine. Actually, I had understood that he thought the he was not really quitting because he was still addicted. I will discuss this with him. I'd like to figure out if he wants to quit the addiction or if he wants to quit the nicotine. I know that he often mentions that his long term goal after quitting smoking is to also quit vaping. However the problem is that he hasn't yet completely stopped smoking.
As for myself, I don't mind being addicted as long as I can manage my addiction. We can be addicted to many things, to music, to food, etc. It's a problem to be addicted only if it consumes one's life or if it is harmful. I'm just so glad that the health effects of nicotine absorbed by vaping are considered benign compared to the health effects of smoking. And also I now prefer vaping anyway, to the point that I wonder what I would do if I would ever learn that vaping is actually more harmful than smoking. I'm not sure that I would be glad or willing to switch back to smoking!