??? Gravity readings should be the same to keg as they are to bottle... You should make sure FG has been reached in either case. It is possible, but definitely not the easiest or preferable method to bottle from a primary fermenter. To do so, he will need to add his priming sugar directly to the bottles and not the carboy. He can cold crash for 24 to 48 hours to clear the beer as well as firm up the trub/yeast cake without worries.
If it was my friend, I would first tell him to take gravity readings several days apart to make sure FG has been reached, then go buy a bucket ($5 from HD if a 5gal would be enough for him).