I\'ve got a meteor collection on the client side
Friends = new Meteor.Collection(\"Friends\");
Meteor.subscribe(\"Friends\");
I have a user
You need that Collection declaration on both the client and the server.
// common code, do not put under /client or inside Meteor.is_client test
Friends = new Meteor.Collection("Friends");
If you want to use Collection only on Client side and you don't need to save that data to server you can declare your collection in "client" folder or in .isClient() function by passing null to the constructor like this:
if(Meteor.isClient()){
// Some other code
...
onlyClientCollection = new Meteor.Collection(null);
// Some other code
...
}