Suppose your friend is indifferent to the bundles (5, 6) and (6, 6). Are the preferences of your friend monotonic?
In the given case it shows that the preference of my friend is not monotonic. As he is indifferent towards both the bundles means both the bundles give him same level of satisfaction and so he has assigned them the same rank.
According to the monotonic preference second bundle would have been preferred as it contains more of both the goods.