What type of bike does he have, how many miles per week, average mph, and is it mostly paved road, hard packed trail, single track, or down hill?
I commute to work and trail ride with my 4" fat tire ebike about 45-75 miles per week. So far, every mis-hap on the trail was usually 100% my fault. My worst spill so far was when I flipped over the bike on a muddy turn, ended up snapping 2 of the bolts and bending a 3rd that secure the handle bars, landed on my left shoulder blade, and my head bounced of the dirt. I know the helmet saved me from a concussion and my Osprey backpacked acted like an airbag to cushion my fall.
I haven't had any spills work commuting; but, I've had a few close calls. Every single time it was from a distracted driver zoning out, they were on the phone, or the driver was assuming they had the right away because they had the bigger vehicle (rolling stop signs or traffic circle stuff). Adding in road and trail accident together mouth/teeth injuries are extremely low % compared to head, limbs, and torso.
I would lean toward making sure he is visible 1st and protected 2nd if the worst happens.
- good LED headlight that can be seen day/night in any mode. Distracted drivers will see a blinking LED light better than a solid light in the day (I use Niterider Pro 3600). The good thing about this light is it cast a extremely bright wide and long beam down the road. The light is bright enough at night to be seen from the rear and my bike silhouette is clearly outlined for added safety. I just need low for commuting and use med or hi for night trail riding.
- blinking rear tail light that can be clearly seen during the brightest daylight. I use Light and Motion Vis 180 and it has an added feature of a yellow follow light you can switch to if he likes to ride in a pack (so he doesn't night blind the bikes in the rear).
- helmet light (Niterider Pro 2200), nice to have a back-up if primary fails. I also use my helmet light when street riding to see around corners, highlight/avoid road debris/potholes, and to shine oncoming cars to get their attention at intersections if they might do a rolling stop.
- helmet rear light, I use the same Vis 180 tail light and mount it to my Fox helmet.
- bright neon colored riding gear with reflective areas (neon for day, reflective for night). Helps with the side view since most bike are very hard to see from that angle at night.
- he might want to add reflective tape to help with the side view if it doesn't take away from the bike's aesthetics.
If he is a road bike person that travels +20 mph on average. I would think about padded leg and arm protection to help with spills that might result in road rash. The protection will also help limit sunburns and UV exposure if he sweats off the sunblock.