if Grab employs the same algorithm as uber, then u will not necessary get your friend even if you're next to him.
airport rides takes into considerations how long he's been online for, how many others around etc. so, another driver nearby who's been given short trips for a while on that might get it instead of your friend who's just came online.
not to mention Grab might suspect you guys of 'gaming' and impose a ban.
just do an offline deal if he's your friend.