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First paragraph. Say::
Mats sat in the cafeteria and DRANK coffee with his colleagues.
It is highly likely it is the same cafeteria every day, thus we use "the" there.
Next sentence. Say:
The anger was building up inside him as he listened to them discuss the weather, TV programs, restaurant food, and other trivia.
Since we know who "the others" are it is more natural to use the word "them" (which clearly refers to his colleagues). Because we already know who we are talking about.
Student or Learner