I think there's multiple reasons. For starters, man is often used to symbolise "person" or even "humanity".
More specifically, I think in duolingo exercise there was a sentence something like "Это их человек", which is used to refer to a person who acts on behalf of and is loyal to 'them'. In English, you'd use "man" instead. - "They are Al Capone's men / goons."
So bottom line is, I don't think человек necessarily ever implies male, it's more that sometimes the English sentence prefers to use the word "man" instead of "person".
Edit: In this case, I don't know why they'd choose "man" over the regular translation. Perhaps because it says "sees HIS father", but as others have pointed out, the sentence means 'This person sees "SOME OTHER MAN'S" father'. As such, 'Эта женщхина видит его папу" should be correct, so there's no reason to think that человек should be a man rather than a woman.
What I find most curious about this situation - as far as I've been able to gather - is that masculine nouns ending in -a or -я are declined as if they were feminine, but the adjectives and possessive pronouns and determiners (these, those, etc.) attached to them as declined in the masculine. thus "my dad" is мой папа not моя папа. "I know my dad" is я знаю моего папу, where "my" is declined as Accusative Masculine Singular and "Dad" is declined as Accusative Feminine Singular.
I have not seen this kind of relationship in any Romance language.
Jeffrey855877, Practically all Romance languages lost the oblique cases (they usually retain the nominative or accusative latin case, used as a single one) but they do have similar oddities as well. Think in particular of Italian: l'uovo (the egg), singular masculine, becomes le uova (the eggs) plural feminine. And in Italian there are words ending in "a" which usually denote a feminine noun but which are actually masculine so that you have to use the masculine adjective form with them (e.g. "il completo marasma"/"i completi marasmi", "il primo vaglia"/"i primi vaglia" - from which you can also see that some decline as masculine in the plural form and some do not - at all). :D
Контекстом, подбирая другие формулировки.
Тут была похожая история с "бабушкой". "Мальчик объяснил бабушке что-то там". Без контекста кажется само собой разумеющимся, что это его бабушка. Если бы чужой бабушке объяснял, так и было бы написано или из контекста было бы очевидно, что речь идет о бабушке другого ребенка.
owtaylor: "Another way of putting it (for those who don't know Latin) - nouns that are end in -а but refer to masculine people (папа, дедушка, etc) have the same forms as any feminine noun ending in -a as far as the word itself goes, but the rest of the sentence treats them as masculine.
старый дедушка - an old grandfather - not старая дедушка
папа купил хлеб - Dad bought bread - not папа купила хлеб"
It's папу instead of папа because we need to change the noun to the accusative case. We always put the direct object of a transitive verb into the accusitive case. Because we are saying "This man sees his father", "his father" is the direct object so we must put it in accusatice case, hence: Этот мужчина видит его папу.