That particular issue is fixed in upstream's 0.7.0. I patched it, but hit another issue afterwards:
=================================== FAILURES ===================================
_ test_google_docstring_help[simple_google_docstring --help 2 --arg2=test-usage: example.py simple_google_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n  arg1         Description of `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n] _

args = ['simple_google_docstring', '--help', '2', '--arg2=test']
result = 'usage: example.py simple_google_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n... `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n'

    @pytest.mark.parametrize('args,result', GOOGLE_DOCSTRING_HELP_CASES)
    def test_google_docstring_help(args, result):
        args = args.split()
        with pytest.raises(SystemExit):
            with capture.capture_sys_output() as (stdout, stderr):
                program.execute(args)
>       assert result == stdout.getvalue()
E       AssertionError: assert 'usage: examp...n of `arg2`\n' == 'usage: examp...n of `arg2`\n'
E         Skipping 146 identical leading characters in diff, use -v to show
E           1`
E           
E         - options:
E         + optional arguments:
E             -h, --help   show this help message and exit
E             --arg2 ARG2  Description of `arg2`

mando/tests/test_google.py:57: AssertionError
_ test_numpy_docstring_help[simple_numpy_docstring --help 2 --arg2=test-usage: example.py simple_numpy_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n  arg1         Description of `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n] _

args = ['simple_numpy_docstring', '--help', '2', '--arg2=test']
result = 'usage: example.py simple_numpy_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n ... `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n'

    @pytest.mark.parametrize('args,result', NUMPY_DOCSTRING_HELP_CASES)
    def test_numpy_docstring_help(args, result):
        args = args.split()
        with pytest.raises(SystemExit):
            with capture.capture_sys_output() as (stdout, stderr):
                program.execute(args)
>       assert result == stdout.getvalue()
E       AssertionError: assert 'usage: examp...n of `arg2`\n' == 'usage: examp...n of `arg2`\n'
E         Skipping 145 identical leading characters in diff, use -v to show
E           1`
E           
E         - options:
E         + optional arguments:
E             -h, --help   show this help message and exit
E             --arg2 ARG2  Description of `arg2`

mando/tests/test_numpy.py:62: AssertionError
=========================== short test summary info ============================
FAILED mando/tests/test_google.py::test_google_docstring_help[simple_google_docstring --help 2 --arg2=test-usage: example.py simple_google_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n  arg1         Description of `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n]
FAILED mando/tests/test_numpy.py::test_numpy_docstring_help[simple_numpy_docstring --help 2 --arg2=test-usage: example.py simple_numpy_docstring [-h] [--arg2 ARG2] arg1\n\nExtended description.\n\npositional arguments:\n  arg1         Description of `arg1`\n\noptional arguments:\n  -h, --help   show this help message and exit\n  --arg2 ARG2  Description of `arg2`\n]
========================= 2 failed, 77 passed in 0.20s =========================


Which seems to be https://github.com/rubik/mando/pull/54