What can I do to help my friend who is in the U.S. illegally and has been detained by law enforcement?

Unfortunately, there is probably not a lot you can do to help him. When a person enters the country illegally and stays for more than 1 year they are banned from returning to the country for 10 years. Even if this person had a lawful means to immigrate to this country (i.e. close relative who is U.S. Citizen) he would have trouble because of his unlawful presence in the U.S. and because he is now in removal proceedings.