Originally Posted by Often1

Please confirm that AA operated the ATL-MIA segment. If some other carrier, substitute that carrier for AA.

Under the Montreal Convention, AA (presuming that it was the final delivering carrier for ATL-MIA) is liable for reasonable expenses up to a maximum of approx GBP 1,260. That is the sum AA would be liable for if the bag is lost. Loss is declared on the 21st day.

Your friend should simply keep the purchases reasonable and keep receipts. Easiest method is to photograph each item with its receipt.

If the bag is lost and never recovered, the amounts paid out for interim reimbursement are deducted from the 1,260.

Additionally, some travel insurance has tight notification deadlines. Make certain to file a notice of claim even if a claim is never filed.