"who honestly believes that emotions are unnecessary and harmful"
This is really hard. If the adult is a parent - I think you have to tackle this, and - Spock-like - appeal to reason.
Accept for the sake of argument that emotions are unnecessary and harmful. Nevertheless, emotions are real, and often biochemically driven. They play a profound role in shaping behavior, and evolution made it so. There is a survival imperative served by "fight or flight." There is a survival imperative served by love. There is a survival imperative served by empathy.
So emotions and emotional responses are hard-wired. They exist. You can't ignore them away. In fact, ignoring them will just RAMP THEM UP in response to the survival imperatives they address.
As MoN says, nothing will happen until this person is ready to change. And this person will reject emotional pleas about the child's needs. So it seems to me the focus has to be on unemotionally intellectualizing the reason that change here is a good idea.
Also - this is easier to say than do - but I'd try to keep foremost in your mind that this adult is likely wildly afraid of emotions. Just guessing. But I'd put money on it.
