I too feel that an apology is in order. An apology for the way it was said, not necessarily for the content. You are entitled to the way you feel. Hiding our feelings on this distructive cult helps no one.
How about after the apology, invite your son to accompany you to the hospital. If he declines, let him know you respect his feelings and his right to shun his former friends. You do not have to agree with it, but at the same time you want to show respect for his beliefs. (no matter how cokeyed they are).
Best wishes for a peaceful outcome to this tragedy.
Loris