"This is my coworker."
Translation:To mój kolega z pracy.
Yes, it is. Polish word "kolega" has a broader meaning that English collegue or co-worker. If you use "kolega" without the addition "z pracy" it could mean any kind of acquaintance ranging from your co-worker, that you barely know personally to your friend. There is a quite funny tendency among some Polish native speakers to actually use "kolega/koleżanka" instead of "przyjaciel/przyjaciółka" (friend) with no meaning change. Some use it interchangeably .
My father used kolega when speaking with his army buddy of many years - using it instead of the more formal "Pan Stas" - Even though they were best friends for decades they never used each other's first name without one or the other title.
Yes, "kolega" means not very close friend. It could be somebody with whom you hang out from time to time. I'd say "kolega z pracy" means you know this person only from work. Sometimes you don't even know him personally.
Co-worker translates directly to 'współpracownik', but means also 'kolega z pracy'. And 'kolega' may be also from the gym/school/etc.
'To jest mój wspólnik' is not accepted but I thought it's a synonym with współpracownik