You can type in your friend's ( or anyone's) screen name in the search field at the top right corner of any page. It won't bring up everything but it will show most of that person's recent activity in posts, journals, the stir and answers. For other things like photos, journals, etc on your home page is your recent activity widget. You can adjust it to tell you what your friends are up to. It will tell what groups they join, when they change their avatar, when they make new friends etc. Just hit edit in the widget corner and adjust your settings.
One other note, just because you can bring up the activity, doesn't mean you can access it. If it is in a group that you don't belong to or it is set to private, you may get a link but not be able to see it.
at 3:29 AM on Apr. 17, 2010