Yeah - that's a problem. To Eddie - he's using the AS3 library, which accesses the REST API through an OpenSocial implementation - there is no OpenSocial function which takes an arbitrary Id to collect information (only the constants VIEWER and OWER), so there is no function in the AS3 library, either.
About this: http://api.msappspace.com/opensocial/v1/VIEWER/friends/profile.xml
Where did you find that endpoint? I've never seen the ability to collect "profile" through friends of a user. The closest endpoint I found like that is this:
http://api.msappspace.com/opensocial/v1/{userId}/friendslist/{friendId}.xml
But that just returns the same Basic information set as any friends' request (the exception being that you can get a specific friend by Id, instead of having to page through returns until you find the friend you want).
What Eddie means when he says they must have the App installed is that you can then use their Id in place of VIEWER; but as I said, that functionality isn't available in the AS3 Library, as far as I'm aware.