This is unlikely to be of use to your friends. Companies hide these job openings because they aren't real: they are filled by a real person right now. If someone applies, they won't be hired because there's no extra headcount. They will just be rejected after a resume review. Companies usually don't even extend interviews to such candidates. Applying only delays the green card process of a foreigner since they will need to rewrite a job description to be even more tailored to that already employed person.