First off, he/she did not contradict themselves, you are simply treating Kantian ethics as if they are the only valid ethical system in the world. "If everyone did it shows that it is an unethical practice" is an application of Kantian ethics, more specifically the formula of universality/the categorical imperative, which certainly isn't the only valid view of ethics and morality that exists in the world.
If you are a strict adherent of Kantian ethics, the following must be true: It is always unethical to lie. What if you lived in a society where petty theft was punishable by death? Is it unethical to lie about someone stealing $5 in order to save their life? Based on Kant's categorical imperative, he would say that it is. If you believe in your original statement, that "if everyone did it it shows that it is an unethical practice," it follows that you, like Kant, would say that lying in this instance is unethical, as lying is always unethical.
I think we can both agree that that is ridiculous and the context of a situation certainly should play a role in the morality of a decision, suggesting that Kantian ethics are, to a degree, flawed as a complete system of ethical reasoning.
Long story short: the only way for what you claim to be true is if Kantian ethics are the only valid ethical principles, which is not only close-minded but arrogant.