The burden of proof is on the one claiming that it's a god. They can keep believing it's a god if they want, but it's in no way up to anyone to prove that they didn't see god. That's not how the burden of proof works. The most that anyone could do is try to explain to them why believing it was god with absolutely no evidence is not a good thing and why they should care if the things they believe are true and not just if they make them feel good.