colleague also means "kolega"

It is just that the people you went to school with, or to university, people you were at sooker team, kids from neighbourhood you used to play with, people you work with are "koledzy"

and real friends, that you love with friendship love - those are "przyjaciele".

And facebook friends are "znajomi" - people I've met, people I know, acquaintances. (out of my 200 znajomych on facebook I would call "koledzy/koleżanki" about 40 and "przyjaciel" only 1)

In Poland "przyjaciel" (friend) is someone really necessary for you; you have only a few of them. For example, on Facebook there are "friends", but in the Polish version - "znajomi" (acquaintances). By the way, the singular nominative form of "kolegów" is "kolega".

First one: "her friends" are only girls, the second one is either that these friends are only boys, or it's a mixed group.

Although actually for this very specific word somehow it sounds to me a bit weird to include a mixed group in 'koledzy'. I think I'd say: Moja siostra wielu kolegów i koleżanek". Although grammatically, the masculine personal plural includes mixed group of people.

Depending on the gender of what it describes.

'wielu' is masculine personal, 'wiele' is not masculine-personal. So here you can either say "Moja siostra ma wielu kolegów" (mixed group of colleagues) or "Moja siostra ma wiele koleżanek" (if they are all girls).

https://en.wiktionary.org/wiki/wiele#Declension

"many" is accepted, it should have worked.

I can perhaps imagine someone saying that to their parents (pretending it's not anything serious), but basically that's an equivalent of calling your boyfriend/girlfriend just "a friend". A personal choice, there isn't anything remotely romantic in the word itself.

"kolego" is Vocative (when you're addressing someone), so I guess you meant "kolega".

sure