[xigoi]

You don’t need to know the digits of a number to prove that it’s irrational. You just need to demonstrate that it can’t be expressed as the ratio of two integers.

[IngoBlechschmid]

Indeed! For instance, Wikipedia presents a couple of proofs of the irrationality of √2 (including a geometric one). None of these require knowledge of its digits.

https://en.wikipedia.org/wiki/Square_root_of_2

[pfdietz]

I think the person you were responding to doesn't understand proof by contradiction.