'Hen' is the personal pronoun for 'them' when it's an object, 'hun' when it's an indirect object (and 'hun' can also be the possessive pronoun). Probably half of the Dutch people make this mistake too though :-)
I agree Heather! Im struggling with a few if these, it is good to know this is the sort of thing Dutch speakers correct eachother on. Good to see its not just English speakers who have the grammar police!