if you mount it horizontally it will make your plants grow sideways.
not really the light emitted will be the same no matter if its upside down horizontal or vertical . I belive your friend means the plants will be exposed to more light if the bulb is mounted horizontally over them.
Eclipse system 12
AH 36w retro kit light 2pm-11pm
Pressurized Co2 with Red Sea 500 Reactor
10g shrimp Only tank
28w corallife aqualight
whisper 20 w/ sponge pre-filter.
Feeding HBH Lobster crab bites and Algae Wafers.
No Co2 and very light fertilization
There are some compact screw in bulbs that are mushroom shaped, with a large diameter. Those are made to be vertical above the surface to be lighted. The ones made for household fixtures are usually more cylindrical, and look like they have less restrike if mounted horizontally.