Yeah, but your premise is useless. He invented a new language that only he and the second party knows. The way to crack this code isn’t to crack this by frequency of any letters or letter combinations alone, but rather comparing character frequency to all known languages, finding out what family of languages this constructed language is based on and decipher some of the cognates and frequently used words in that constructed language.
It won’t be a perfect cracked code, but you may be able to pick off a few simple words via cognates and frequently used words. All constructed languages are related to some language family or the other.
That is meaningless. Your code or language has to have some equivalent of the word "horse" or you can't send a message about a horse. It doesn't matter if your equivalent of horse is "887934" or "shark" or " ┤»╝┐". If you want to send one, or at most a very few, very short, messages, then your code will be unbreakable, as the cryptoger will not have enough material to find a pattern. If you send a lot of traffic, then you leave a lot of evidence for a pattern to emerge.