Is it true that you cannot con an honest person?

In general, that's true. Most people are hoping for an unfair advantage, or the unearned, or the not deserved.

Sometimes there are honest people who are scammed. Notably, those trying to get their home's refinanced.

I've sympathy for someone trying to pay a mortgage and being fooled by a bogus lender. But I've no sympathy for someone hoping to have a Nigerian hide oil money in his personal bank account so he can have a 10% cut of stolen money.