The best thing you can do to help your friend in the situation you describe is to help him get or pay a lawyer, if that is an issue for him. Then, once he has a lawyer working on the case, cooperate with the lawyer and answer any questions his lawyer might have about what you know.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney in your area who regularly practices in the subject matter which your question is about. You should develop an attorney client relationship with the lawyer of your choice so that your communications will be subject to the attorney client privilege and have the other benefits of a professional relationship. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific matter as partially described in the question.
My goodness. Helping your buddy means one thing . . . help him find counsel. The allegations are extremely serious and there is no way this fellow takes this on alone.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Your friend needs an attorney that specializes and has represented parents/others in actions against DHS. If he already has counsel, then you can be supportive and instrumental in any hearings that DHS brings, this could include being a character witness. After a decision is made and if it is against your friend there is an administrative appeal process. If you believe that your friend is being targeted I would recommend consulting with an attorney to determine if there are actions that could be brought against DHS or the caseworker.