I have not heard this and I personally wouldn't bother to do it cos as Leasha said it is likely to cause more problems than it is worth!
I was encouraged that for daytime naps the curtains should be allowing a bit of light it. I believe it gives them a cue to know that it is just nap time and not bed time. Flynn's room is on the dark side of the house but I always have the curtains open for day sleeps to let him know that it is daytime and just time for a little sleep (that is what i always say to him when it is nap time).
Some kids are better sleepers than others and your friend may have had an excellent sleeper without blacked out windows as much as with them!
HTH