CheckWordIsInTiles

 private static bool CheckWordIsInTiles(string Word, string PlayerTiles)
        {
            bool InTiles = true;
            string CopyOfTiles = PlayerTiles;
            for (int Count = 0; Count < Word.Length; Count++)
            {
                if (CopyOfTiles.Contains(Word[Count]))
                {
                    CopyOfTiles = CopyOfTiles.Remove(CopyOfTiles.IndexOf(Word[Count].ToString()), 1);
                }
                else
                {
                    InTiles = false;
                }
            }
            return InTiles;
        }

The input PlayerTiles is a string representing the 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 actually spelling it.
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 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.