They can't see well in the dark, they kinda zone out at night, but that's no big deal. The only reason to add a light in the coop is to stimulate longer days and more egg production. I do not light my coop.
10 hens is about the correct ratio per rooster, maybe that's what your friend was thinking?
I have 13 birds on my yard right now, and tons of people have lots more than that. No real reason to split them up, unless maybe it's somehow easier for you to build 2 coops???