This is actually pretty simple.
While in game and go down to sharing. An option to share collectibles will be there which goes onto a list of your friends. Just select the friend you want to give it too and huzzah! They get an achievement.
What that means is you need to have your friend share things with you back. Hopefully if you send first, they'll be nice enough to return the favor but that's not always the case. It's best to ask someone to share with you if you can.
EDIT: As per request; collectibles generally are statues, fountains,other things, etc, that you find/gain access to throughout the game in some way or another. They are, from what I understand, the items that have a yellow background (for the most part). You can see all of them in the same menu you do this achievement in. Collectibles you don't have will be grayed out while others will be lit up.
Something to keep in mind as well, it doesn't matter if you or your friend has the same things you do. As long as a friend shares with you, you're fine.