In #1, it is the co-workers, not she, who remain as good friends. She cannot be "friends" because there is only one of her: "Her co-workers remain good friends." Notice that both nouns are plural. #2 is almost right but "with" needs to be replaced with a different preposition: "She remains a good friend for (or "to") (each of) her co-workers.

Hey :) As a fellow non-native speaker, I'm wondering if it's either "She remains good friends with her co-workers." or "She remains a good friend to her co-workers." Such a good question you asked, and I'm also interested in the correct answer 🙃

I think both work, but 1. sounds more natural to me, a native speaker. I appreciate Jonathon's point about singular/vs plural, but I hear sentences like "she is still good friends with him," all the time, whereas I almost never hear "she is still a good friend with him". I think it may be an odd construction where 'friends' refers to all the parties. Or it could be just one of the technically incorrect things that has become common usage.