Either a 50/200 or a 100 yard zero is the most practical. I prefer a 100 yard zero for my specific needs as it gives me a flatter trajectory throughout the ranges I am likely to need to make engagements (urban patrol rifle duty).

If your friend will regularly shoot out past 200 yards then the 50/200 yard zero might work better for his needs.

He can get a rough zero for either distance @ 25 yards
For a 100 yard zero have him adjust his point of impact to 1.5" below his point of aim. Then he must confirm it @ 100 yards and make any necessary adjustments to get him dialed in at 100 yards.

For a 50/200 yard zero have him adjust his point of impact to 1.15" below his point of aim @ 25 yards. He must then confirm at 200 yards and make the necessary adjustment to get dialed in @ 200 yards. He can then come up to 50 yards to see where his point of impact is. If he is properly dialed in @ 200, then he should also be on @ 50 yards, or no more than an inch off.

Either method is the most practical zero for the effective range of the .223 AR platform rifle/carbine. It is critical that the shooter confirm his zero at the actual desired distance, and not just rely on the rough 25 meter/yard zero.

I also recommend shooting 5 round groups between adjustments.

The true benefit of the 100 yard zero is that from 0-200 yards you only need to take into account about 2.5" of total trajectory variation, and the bullet path never travels above the line of sight.

Your zero, and understanding it is the basis for everything you can do with the carbine. Don't skimp when it comes to zeroing. Take your time and get it right. Reconfirm your zero as often as you can. It typically takes us the better portion of the entire first day of a AR users course to get brand-new AR shooters dialed-in with their zero.

Hope this helps....

ETA: I DO NOT recommend following Military doctrine to zero your AR rifle/carbine. My personal opinion is that the Army's 25m zero process is partially responsible for the "poor stopping power" reputation that the .223/5.56 has received over the years in combat. This is mostly due to the trajectory of the 25m zero. It requires too much "hold-over" consideration to take into account. For example: with a 25m zero, if you need to make a shot at a distance of 200 yards, the shooter will need to hold their point of aim approximately 10 inches over their desired point of impact to get a accurate hit. At 100 yards they still need to hold 6+ inches over their desired point of impact. That way too much for someone to think about under a timer, let alone under fire in my opinion. The military gears their zeroing process towards the "Lowest common denominator". They train tens of thousands of new shooters every year, so they need to expedite the process as much as they can. Unfortunately, it often doesn't translate well in the field.