She could do several things actually. First thing is to find the root of his fear. Is is just the dark? Are there monsters in his room? Are the shadows coming for him?
-let him sleep with the light on
-Buy him a cool looking night light, lamp, or those motion lamps that put pretty designs on the ceiling. Even a lava lamp could work
-Give him a stuffed version of his favorite superhero to sleep with. (My 5 year old sleeps with a giant Spiderman)
-another mom mentioned flashlights
-Have mom or dad "check" the closet and under the bed for monsters before tucking him in at night. My husband actually went into the closet and "beat the monster up" to staisfy our sons
-But blackout curtains up on the windows to keep the shadows out
-if possible, re-arrange the room so his bed is as close to the door as possible, so he can feel like he has a quick escape route
-don't ever chastize him for being afraid
-buy him a radio/cd player so he can listen to music while drifting off to sleep. That'll help minimize the "scary sounds" of a quite house.
-If he gets up in the middle of the night, cuddle him until he's calmed down, then walk/carry him back to bed. Repeat as needed.
-be patient and consistant!
I know this is a bit long, but there are numerous reasons for a kid to be afraid of the dark, and she needs to find out exactly what it is about the dark that scares him. Maybe taking him for a walk at night to show him how beautiful it actually is could help as well, she can point out the nocturnal animals, the moon, the stars and other things for him to see.