In a code, each letter shifts 2 places forward in the alphabet (A→C, B→D...)

In a code, each letter shifts 2 places forward in the alphabet (A→C, B→D...). Coding "WY", Disha thinks the rule breaks down at Y, since only Z is left after Y, not 2 letters. Her brother says there is a clean answer for this too — the alphabet wraps back around to A once it ends. Who is right?