Solution

[Edward_Feldek]

Header:

class FBullCowGame
{
private:
	bool IsLowercase(FString) const;
};

FBullCowGame.cpp

EGuessStatus FBullCowGame::CheckGuessValidity(FString Guess) const
{
	else if (!IsLowercase(Guess)) // if the guess isn't all lowercase 
	{
		return EGuessStatus::Not_Lowercase;
	} 
}

bool FBullCowGame::IsLowercase(FString Word) const
{
	for (auto Letter : Word)
	{
		if (!islower(Letter))
		{
			return false;
		}
	}
	return true;
}