The problem is the same using http://api.hi5.com/rest or http://api.hi5.com/json. The result is a 200 code with a blank body:

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/json;charset=utf-8
Content-Length: 0
Date: Thu, 06 Nov 2008 06:44:51 GMT

It should be either XML or JSON content containing the list of friends in parameters.

Some references from people having the same problem:
http://www.hi5.com/friend/group/2364084--10108154--Hi5%2BPlatform%2BDevelopment--Ac--topic-html
http://www.hi5.com/friend/group/2364084--14880739--Hi5%2BPlatform%2BDevelopment--re--topic-html
http://twitter.com/hi5status/status/942642383