I read in Serebii that you got the third Pokemon if they defeated the Elite Four.
Also, if the friend is online, you can get the hidden ability of the Pokemon.
I read that serebii thing days ago and its wrong. Several of my friends beat the elite four and i only see 2 for them.
EDIT: Now that i think bout it though, i havent seen them online since they beat the E four... maybe you have to beat them and be online.