CheckWordIsInTiles

 private static bool CheckWordIsInTiles(string Word, string PlayerTiles)
        {
            bool InTiles = true;
            string CopyOfTiles = PlayerTiles;    //Sets CopyOfTiles to equal PlayerTiles.
            for (int Count = 0; Count < Word.Length; Count++)    //Checks how long word is and if it's larger than Count.
            {
                if (CopyOfTiles.Contains(Word[Count]))
                {
                    CopyOfTiles = CopyOfTiles.Remove(CopyOfTiles.IndexOf(Word[Count].ToString()), 1);    //If CopyOfTiles word length
                }                                                                                          equals Count then the word is
                else                                                                                       removed from the index.
                {
                    InTiles = false;
                }
            }
            return InTiles;
        }

The input PlayerTiles is a string representing all the tiles a player currently possesses.
CopyOfTiles is a local copy of PlayerTiles created so that PlayerTiles is not permanently affected, as the purpose of CheckWordIsInTiles is only to check whether the player has sufficient tiles to spell a word rather than to actually execute the player's turn.
The other input, Word, is a string representing the word the player is trying to spell.
The for loop increments from the Word's first to last letter inclusive. For each letter of the Word, the if statement checks whether that letter is contained in CopyOfTiles.
If true, one tile representing that letter is removed from CopyOfTiles as each tile can only be used once.
If false, the Word cannot be spelled with the tiles the player currently possesses, so InTiles is set to false which will later be used to invalidate the player's turn.