Good luck. He's been told all this before, and he just claims that whatever changed the way one method works changed all the others as well in just the right way to make it all look like an old earth. Appeal to coincidence is all he has.
Yes, it’s a coincidence, it’s the anthropic principle, “if we wouldn’t be living in a world where isotopes show an “old earth” we wouldn’t be wondering about them”
You see, we live in a multiverse all showing different rations (and different ages) of isotopes……….we happen to live in a universe where the ratios show a 4.6Byo earth (despite the earth being 6,000 old)……. Given that there are probably potentially infinite universes (and potentially infinite earths) there is a warranty that some of these universes would be 6,000yo despite them showing an ratio of isotopes that seems to show a 4.6Byo age.
Most universes have an age consistent to what the isotopes show, but we simply happened to live in a universe where this is not the case simply by chance.