My method for this function. On the basis of islower() being suggested, I wanted to dig around a bit and understand everything as best I could. This led me to the range of case functions, including isupper(), toupper(), and the tolower() function previously used. Rather than asserting the negative (!islower()), I just went with a check to see if anything in the range was uppercase and if so returned false.
Nothing fancy here, it just represents a bit of added effort.
https://gist.github.com/92408512049/649df2dc4fe8ac82eeec8b5d1cf0d2b9
A note - if Guess is 1 letter then this function will return true regardless of the letter’s case. And if it is empty I think it should return false instead of true 
Yes, it returns true regardless of case if Guess is 1 letter, because it then passes that entry through to the CheckGuessValidity() error handling further on which reminds the player to enter an appropriate length isogram
I meant it should not have been the case. Of course it may work in your specific code and that’s fine but in general each function should do what their name suggests. https://en.wikipedia.org/wiki/Cohesion_(computer_science)
Anyways, good job trying out new things