How do falsetto countertenors work?

Okay, so i have a friend who's a lyric baritone, his voice ranges from A2-B4 in normal voice, with vocal fry he reaches down to an E-flat (barely audible). His tessitura(comforst zone lies within Eb3-Eb4). His falsetto goes up to a D6, and he can sing comfortably in his falsetto up to a G5. He wants to know if the same principals used to classify female voices can be used to classify falsetto countertenors. For example principals like: tessitura, timbre, strength in the middle part of falsetto(like mezzo-sopranos) or the upper part(like sopranos) and/or vocal weight.