With the help of asifrc and TommyBs I found a way to do this in FQL using this query :
SELECT name
FROM user
WHERE uid IN (
SELECT author_uid
FROM location_post
WHERE distance(latitude, longitude, "48.856614", "2.3522219") < 5000
AND author_uid IN (SELECT uid2 FROM friend WHERE uid1=me()))
It's pretty much getting all my friends, searching all their posts close to the center of Paris, then getting their names.
If anyone knows how to do this in Graph API, please let me know, because it's still not a good idea to rely on FQL, I think.