What’s he riding and for how long and what’s his main light?
3 main things to consider (beyond build quality etc)
1) Amount of light needed
2) spread of light needed
3) How long for
1/2 depend on main light and 1/3 depend on battery
Good batteries are a commodity item… and on a helmet you have to balance weight more than bar mounted if you want 1/2 decent run times with 1/2 decent light… or have external batteries in a pack
I’ve botched about and used cheap light then had to buy better batteries etc. but ultimately found something decent on sale was a better buy in the end. If you have a good main light with good reach and spread then you can get a pretty cheap helmet torch as its only really for looking round corners…