The Yedda Twitter API wrapper allows you to call the "Follow" method, so if you have a certain user's credentials you can use these credentials to make that user follow another user.
Mind you that the Yedda Twitter Library uses the old auhtnetication method for Twitter which is unsecure since it requires you to get a user's username and password. The new API uses the OAuth standard which is much more secure since it does not require the user to turn over any details that may tell his/her password.
