Treat it like a normal opiate addiction, because it can have very similar symptoms of withdrawal. If they quit cold turkey, have them schedule it for a time when they have a full week they can take off if they use a lot of it. If they taper off it, have them make a spreadsheet and really plan it out. Tell them to get a scale and start weighing exact doses.

It might not be so bad for them, or it might be hell. Don't underestimate it and plan for the worst. Hot showers help at the worst of it. A heating pad can help a lot too. It's not that you're cold, it just helps your muscles relax. It will make them feel a lot less like their bones are on fire.

Be prepared for insomnia. Drink lots of water, eat healthy. Start exercising! The best thing to start feeling better after stopping is exercise. It will make you feel normal again extremely quickly. You won't feel like it initially of course, but as soon as physical symptoms start going away, exercise will help a ton with the shittiness after. I started jogging and my mind state improved almost to normal.

It might not be so bad, or it might be terrible. Just be prepared for the worst and have some ibuprofen ready with a heatpad and schedule some time off work.