Doing ab exercises for mass gains(Relatively low reps would be optimal) will of course help getting visible abs but you need to have low bodyfat%. We are all different thus your friend sees his abs faster than you. It's down to genetics.
If you have a pull-up bar then hanging leg raises, if not then leg raises on the floor.
yes I'm doing that too... chin ups will my legs are in slanting... it also workout my abs.. thanks!