What is a store's responsibility to help a sick customer?

I am so sorry for the situation and I sincerely hope that your brother is in the road to recovery.  The legalities of the situation will hinge on what is known as the "duty of care" that is imposed upon the store owner in this type of situation.   A duty of care can be high in some instances (like for a doctor) and low in other instances.  Certainly there is a minimum standard that the store owner here fell quite short of under the circumstances.  If the matter is actionable, however, will depend on a number of factors, most importantly how the store owners actions impacted on the final result with your brother.  I would suggest seeking legal consultation in your area.  Good luck to you all.