wielu m pers pl (other plural wiele) - many, much, a lot

https://en.wiktionary.org/wiki/wielu

kolegów should be correct provided that you change wiele into wielu. The whole sentence should look like this: Moja siostra ma wielu kolegów.

"friends" is both male and female (koledzy i koleżanki). It should be accepted

We recently resigned from putting "colleague" as the main translation, because it narrows the meaning too much and in a way it can be even considered a false friend.

The problem is that "friend" is very vague from the Polish point of view. It doesn't say much about how close the friendship is. So "koleżanka" is someone whom you (probably) like, but is not among your closest friends (przyjaciele). But it can also be just a colleague from work. Or a schoolmate, or a classmate. Frankly, it's also a rather vague word.