"They do not help their colleagues."
Translation:One nie pomagają ich koleżankom.
"Nie pomagają swoim kolegom" is not accepted! Reported. Favouring the use of personal pronouns is not very clever.
"ich": They (Susan and Bart) do not help their (Martha and Steve's) colleagues.
"swoim": They (Susan and Bart) do not help their (own) colleagues.
Any form of "swój" always refers back to the subject of the sentence.
You help (give help) to a person, so it's the indirect object. It's the same in German.
So here, the verb's demand for the dative overrules the "nie"s demand for the genitive? Is that the case for all verbs that demand a certain case (e.g. być)?
nie's demand for genitive works only for accusative. All other cases stay the same.
Also it does not change a case after preposition.
"swojemu" is for masculine singular. For plural (luckily here, both plurals) you need "swoim".