English is not my native language. In my native language (Italian) "coworker", like many other words, can be both male and female, but to avoid unnecessary redundancy our grammar allows for the male form to be used to include everyone with no distinction (even beyond the two default genders used in the grammar).
So, technically, my use of "he" was more inclusive than you realize.
The parent might be a non-native speaker and might have a mother tongue in which nouns have genders - the co-worker in German is of male grammatical gender.
