Hi, I'd like to help you with your question.
Yes, I can recommend several books for your friends:
Don't Let Your Kids Kill You: A Guide for Parents of Drug and Alcohol Addicted Children by Charles Rubin
My Daughter's Addiction: A Thief in the Family - Hardwired for Heroin by Marie Minnich
Reclaim Your Family From Addiction: How Couples and Families Recover Love and Meaning by Craig Nakken
Beautiful Boy: A Father's Journey Through His Son's Addiction by David Sheff
Loving The Addict, Hating The Addiction: For Christian families coping with drug addiction by Kecia C Sims - I don't know if your friends are Christians but if so, this book would help.
There is plenty of treatment available for families dealing with their child's addiction. Here is an excellent link for families dealing with drug addiction: http://www.helpguide.org/mental/drug_substance_abuse_addiction_signs_effects_treatment.htm
They may already know a lot of the information because of where there son is in the process, but you never know what else might help.
Have they tried Narcotics Anonymous yet? That is a very good place to start for support. Here is a link: http://www.nar-anon.org/Nar-Anon/Nar-Anon_Home.html
In order for their son to recover, they are going to need to face this situation and get behind their son 100% in his recovery. Otherwise, it will be a hard road for him.
Please let me know if you have any other questions or need more information.