KYBO is a common acronym in Scouting.

"Keep Your Bowels Open"

Whether Bouells is a typo or pun I don't know.

Might be "keep the bowels open"? A very medical greeting, so would be fitting here...

Since "Bouell" isn't a word, but it is a name, and "Bouells" is a plausible misspelling of "bowels", it could be a pun connecting the two. (medical bowel care and a person named Bouell)

Unless it's something specific between them, perhaps it means to keep the heart open, to stay in kindness and compassion. Which would be a traditional, positive thing to say to a dear friend at the end of a letter. Or a physician joke between them, alternatively.

https://www.etymonline.com/word/bowel

Bowel seems to originate from old French “Bouel,” which I guess are related?