userFriendsData.js

var friendQuery = models.User.findById(userID);
friendQuery.exec(function(err, currentUser){

	// filter Users by _id using .friends id array
	// $in operator in Mongo allows you to use an array as search param
	// http://docs.mongodb.org/manual/reference/operator/#AdvancedQueries-%24in
	// in mongoose http://mongoosejs.com/docs/api.html#query_Query-in
	var filter = { _id : { $in : currentUser.friends } }; 

	// select the fields you want
	var fields = 'name photo';

	// build and run the query, 
	// you'll get an array of the objects with the fields
	// or you'll get an error.
	var query = models.User.find(filter, fields);
	query.exec(function(err, friendData){
		res.json(friendData); // return json
	});
});