Hi,
Adult bearded dragons that are 1 year or older do brumate/hibernate over the winter. They don't eat much, just hide in a cave or dig a hole in the sand and sleep. They may get up to get some water, so water should always be available.
To meet the hibernation needs of the beardie, your friend should reduce the amount of light from 12 hours a day to 8-10 hours a day. Also, if he does have crickets running around in there, he might want to just leave one or two with the greens and take the rest out so it won't stress the beardie out. That's just in case it decides to get up and eat - but it'll probably take a drink and go back 2 sleep. It's normal and after winter, the beardie will be back on track! 
-----
0.0.1 Bearded Dragon (Cookie Monster)
1.0.0 Crested Gecko (Vas-D)
0.0.2 Fire Bellied Toads (Norman & Forman)
0.0.2 Goldfish (Lenny & Carl)
1.0.0 Black Lab / Pit Bull (Mikey)