[Gud]

Parent didn’t provide an argument and the burden of proof is on them.

[concordDance]

You can and should do better than them, not descend to their level. It's obvious to any reader that the parent was just making a bold assertion.

[xigoi]

How do you prove that something is false if you weren't given a reason why it should be true in the first place?

[krageon]

By saying what you just said. It's already better than saying "No u".