Is the son obligated to guarantee the money he collected from his friends and gave to an investor who defrauded them, with the friends being aware of this, and does the son's taking money from the investor in exchange for gathering individuals for him affect the guarantee?

What you mentioned indicates that your son was merely an agent for the group in paying the money to the investor. An agent is a trustee who is not liable unless there is misconduct or negligence. Therefore, your son is not liable for what happened if the situation is as you described.