I think asking for the volunteer's credentials it's standard practice across the industry.
In my airline we have a policy where the FA must ask the volunteer for credentials before allowing them to assist. If they fail to show any credentials the captain must be notified and he or she then decides if the FA are to allow the volunteer to assist.
If the volunteer shows credentials then they are allowed to take over and the FA are to assist them anyway they can. If the volunteer fails to provide any credentials and the captain decides to allow them to assist the FA is still in charge and the volunteer is not allowed to give the casualty any medication.