My first time posting here. This is my solution 
bool FBullCowGame::IsIsogram(FString Word) const
{
if (Word.length() <= 1) { return true; } //treat 0 and 1 letter words as isograms
TMap<char, bool> LetterSeen; // setup our map
for (auto Letter : Word) // loop through all the letters of the word
{
Letter = tolower(Letter); // handle mixed case
if (LetterSeen[Letter]) // if the letter is in the map
{
return false; // we do not have an isogram
}
else // otherwise
{
LetterSeen[Letter] = true; //add the letter to the map as seen
}
}
return true; // IsIsogram
}