I usually approach this sort of question by modeling people as approximations to ideal agents who reliably act in such a way as to actually optimize the world for their own values.
If I consider a hypothetical person who very closely approximates that ideal, I'd say it's generally+ not at all wrong to enable them to significantly reduce the quality of their own life... they will either do so, or not, depending on their own values.
If I consider the other extreme, a hypothetical person who reliably acts in such a way as to optimize the world for the opposite of their own values, I'd say it's generally+ wrong to enable them to make any effective choices at all.
Either way, their happiness is largely irrelevant to me except insofar as it's subsumed in their values, and whether I benefit from their actions is irrelevant.
That said: obviously I'm more inclined to motivated cognition when I benefit, and therefore need to be a lot more scrupulous about whether my thinking has gone completely off the rails.
There's something to be said for the rule of thumb that if a line of reasoning tells me it's OK for me to act in ways that predictably lower the quality of other people's lives and benefit mine, I should reject that line of reasoning as flawed... not because that's necessarily the case, but because human minds being what they are that's the way to bet.
+ There are exceptions in cases where I think their values are themselves wrong, but I think that's a different conversation.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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