Ask them the same question I suggest you ask yourself to be less wrong: "what evidence or argument would convince you this specific belief is in error or inconclusive?" If the answer is 'I don't know' then consider finding out. If the answer is 'none' then it's not an evidence or argument based choice so evidence or argument won't change anything. If the answer is 'this evidence / argument' then either the evidence / argument is there or it isn't. If it isn't then the claim is provisionally true, and in no way off limits to further questioning.

Decide if your are talking to your friend or if you are talking to the abstraction of 'pseudoscience' and don't confuse the two.