Third and Second person in one sentence referring to "you"?

Is it correct to say: "You are the one who's saying that he can't understand what he wrote, therefore I can't help you."? How understandable it is? A friend and I were arguing about this.

In that sentence "he" refers to "the one". "You" are the noun-phrase beginning "one..." and ending " ...what he wrote". You could replace that noun phrase with a single noun, such as "fool" as in: "You are the fool, therefore I can't help you".

It is perfectly grammatical, therefore.

If you need to ask whether it is understandable, then maybe it isn't; what is understandable is in the mind of the reader, not of the writer. It is certainly a convoluted way of expressing a simple, if insulting, thought.