If a friend used a hair product to add shine but instead lost large patches of hair and developed sores on her scalp, does she have a case to sue for damages?

Your friend can sue the manufacturer of the product and the seller (store where the product was purchased) for negligence and strict liability.  Negligence and strict liability are separate causes of action (claims) in the lawsuit.

Negligence is the failure to exercise due care (that degree of care that in this case a reasonable manufacturer would have exercised under the same or similar circumstances to produce  a product that is not defective).  The defect may be a manufacturing defect (aberration) or a design defect.

The seller is liable even if the seller did not know and could not have known that the product was defective.

Strict liability is liability whether or not due care was exercised.

When your friend completes her medical treatment and is released by the doctor or is declared by the doctor to be permanent and stationary which means no further improvement is anticipated, she should obtain her medical bills, medical reports and documentation of any wage loss. Her personal injury claim filed with the insurance carriers of the manufacturer and store will consist of these items.  Compensation for the medical bills is straight reimbursement.  Compensation for wage loss is straight reimbursement.  The medical reports will document the nature and extent of her injury and will be used to determine compensation for pain and suffering.  Compensation for pain and suffering is an amount in addition to the medical bills.

Prior to filing a lawsuit, it may be possible to settle the case with the insurance carriers of the manufacturer and seller.  If your friend is dissatisfied with settlement offers from the insurance company of the manufacturer or store or both, she should reject the settlement offers and file her lawsuit for negligence and strict liability against the manufacturer and store.  If the case is settled with the insurance company for either the store or manufacturer, NO lawsuit is filed against the party with whom a settlement was reached.  If the case is settled with both manufacturer and store, NO lawsuit is filed.  If the case is settled with the store, but not the manufacturer, only the manufacturer is named as a defendant.  If the case is settled with the manufacturer, but not the store, only the store is named as a defendant in the lawsuit.  The lawsuit must be filed prior to the expiration of the applicable statute of limitations or your friend will lose her rights forever in the matter.