I learned somewhere else that "kolega" is also a polish word for "friend". How true is that?
Kinda true. "kolega" would in many contexts be translated as "friend", but its meaning is quite vague. Generally, "przyjaciel" is rather a closer friend. So "kolega" may be someone that you really like, but he's not among your closest friends. But it can also just be a workmate, schoolmate etc. So it doesn't necessarily have to mean that you even like this person.