Like everything to do with bicycle riding, it depends on the environment. People riding at night through a city have different requirements than people riding on country roads. People who can stick to residential streets during their entire ride have different requirements than people who have to ride on heavily traveled main roads with lots of distracting lighting in the area. People who can take MUPs have even different needs.

The lightest weight system would be one where people are riding residential streets with street lighting, and they just need to be seen. For that, a small flashing LED front and back, bright enough to be seen but not to see by, is probably fine.

If any riding on high speed roads is needed, I'd say the rider should step up to something like at a minimum a 200 lumen flashlight up front and a Cygolite Hotshot or one of the other premium taillights (PDW Danger Zone or Radbot, Cherry Bomb, Mars 4, etc)

If the rider will be in either an area where there are a lot of other lights to contend with and their light might get lost, I think front and back should be beefed up - one or two 400 lumen front lights, probably a couple of tail lights as above or one of the more serious ones like a Dinotte 400.

In dark rural areas (where I ride, so this is the only suggestion I've posted that comes from actual experience), a 400 lumen light is nice if you are on sketchy road surfaces (I ride gravel for a few miles), 200 lumens is find most of the time, and any of the above taillights is fine. I think a strong taillight is good because people are approaching your butt at 60+ MPH, possibly over blind hills, possibly sucking on a coffee or messing with stuff, you want to get their attention ASAP.

Doubling up on everything is nice if you can, for reduncancy. More important on the taillight since if it fails you may not notice. I think you should be RUNNING two taillights at night. Up front it's OK to just HAVE two lights, or maybe even just a spare battery if you trust the light to not actually fail.