The regulation prohibits "messages encoded for the purpose of obscuring their meaning, except as otherwise provided herein" so if you and your friend made up yo

The regulation prohibits "messages encoded for the purpose of obscuring their meaning, except as otherwise provided herein" so if you and your friend made up your language to keep your communications obscured, then it seems that it would not be allowed. If you happen to speak a language that's not commonly known in the area then that would likely be fine.