I’m afraid this isn’t possible with the current version of the plugin and I doubt whether it will be possible with future versions.
The Foursquare API only allows a user who has explicitly authorised access to their checkins via OAuth to gain access to the list of (recent or historical checkins) (see https://developer.foursquare.com/overview/community).
Although the checkins by a user API endpoint at https://api.foursquare.com/v2/users/USER_ID/checkins allows a user id to be supplied, the only valid user id is self which is the currently authenticated user (see https://developer.foursquare.com/docs/users/checkins).
To do what you’re proposing would mean that each user who you are friends with on Foursquare would have to explicitly authorise your WordPress site via OAuth and that a Foursquare API call for each and every user would have to be made. Although not expressly forbidden by the API’s terms of use, I suspect Foursquare would not approve of this. Also, the amount of API calls that would need to be made would place a substantial delay on page load times, which isn’t something you’d want either.
So for now and probably for the foreseeable future, the Foursquare API just doesn’t support this; if this did become possible, it would need a substantial amount of coding changes to the plugin to enable this.
In summary, a great idea but not one that can be realised, for now at least.
-Gary
