Some soft lighting I think is fine if it is geared towards like a night light. Too much light keeps their brain waves in semi non sleeping mode. Light is an indicator in our brains that it is time to wake up, so the full deep sleeping isn't allowed with light in the room. White lights are the worst in preventing good sleep. Red, bluish lights allow for the better sleep. I wouldn't have a lamp on for even safety reasons if that is what you do.
I have the LED soft night lights that basically just lights up the floor area, they provide enough light if someone had to go to the bathroom, for the kids to see the objects in their room to a degree, but they go off with the sun and come on in the dark and are very dim. I think just gauge how light or bright it is and maybe find nightlights appropriate to allow for deep sleep.
All kids have this phase and yes, dark is better for good sleep but they are kids! :) You can educate them about why it gets dark, what happens with the sun and the moon, that helped my kids but kids are apprehensive about what they cannot see. Normal.
Both of mine slept in pitch dark for years, then as they got older requested a dim night light and I feel better as they can see if they have to go potty at night!
Don't worry about a friend and thank her for the advice, you know your kids best and if they are sleeping really well, it isn't a "light" persay then not a problem as far as I can tell! :)