Pattern matching with sigils

I’m not quite sure where you get the is_boolean/1 from. I never used that.

The first step is (as I said already) to simply replace your first condition with my function:

defp check_input(input, game = %State{}) do
  input = String.trim(input)
  cond do
    only_single_lowercase(input) ->
      Map.put(game, :guess, input)
    true ->
      IO.puts("please enter a lower-case letter"
      accept_move(game)
  end
end

The next step is to rewrite the cond into a case (and inline my function):

def check_input(input, game = %State{}) do
  input
  |> String.trim()
  |> case do
    <<c::utf8>> when c in ?a..?z ->
      Map.put(game, :guess, input)
    _ ->
      IO.puts(…)
      accept_move(game)
  end
end

And now it should be easy to extract this 2-clause case into a 2-clause function.