It is not your responsibility to take that position. They're the ones asserting something that flies in the face of most of observable, demonstrable reality. If they can't successfully make their case, that's not our problem.

To answer the question anyway - how can you convince a theist that their position is wrong? That this isn't the right approach. There's two things a person needs to successfully deconvert:

  1. A solid epistemological toolset - so they can accurately assess reality. Being aware of common logical fallacies can help, like some sort of quick spelling/grammar check on your own thinking.
  2. They need to care.

You can help educate a person, piece by piece, on #1, but unless the person cares that what they believe is actually true, and/or that their fundamental approach to knowing reality actually works... you're going to run into into a concrete wall. There's no magic sequence of words that'll work. Most atheists who were former theists, cared. They dug into what was told to them their entire lives, and found it wanting, and stopped being convinced by it.

For many religious, they won't give it up, and sometimes because sometimes it's more practical. The secular part of society doesn't hold a candle up to the long-time organized social infrastructure of the religious... even if we're talking about something as straight forward as daycare. That's improving, but it's going to be difficult to argue someone out of giving up their free daycare, because technically atheists are intellectually right.

Ultimately, if the person does start caring, they'll do your work for you, but it's not something you can just make happen.