Logging security events in MSSQL
You can extract much more data from the event instance in the Service Broker message to store in the table. Specifically, TargetLoginName should bring back the affected user for you, something like this should work:

SELECT @message_body.value('(/EVENT_INSTANCE/LoginName)[1]', 'sysname') AS [ActioningLogin],
  @message_body.value('(/EVENT_INSTANCE/TargetLoginName)[1]', 'sysname') AS [AffectedLogin]


More info here.