How can I advise my friends to wear the niqab?

If your friends are unaware of the ruling on the niqab, you should teach them the evidence for its permissibility and obligation. If they uncover their faces in imitation of those who permit it, then there is no blame upon them. However, try to persuade them to wear it due to the increased covering it provides and to avoid the harm of the immoral, using a good approach, gentleness, and kindness. Allah Almighty says: "Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best." Be keen on increasing their faith and their reverence for Allah's commands, and supplicate frequently for their guidance.