Do they use square? https://squareup.com/us/en/gift-cards

I truly hope your friend can figure out a way through this ... however isn't selling gift cards just kind of punting the problem down the road? If their customers buy gift cards now, and then return to use then in X weeks/months, wouldn't they just be working for "free" at that point (assuming all the funds from the gift cards were used to get them through the customer-less time)?

It effectively acts as an interest free loan to the business.

Off the top of my head I would say open a Shopify store and find a module that creates a unique order code(Assuming Shopify doesn't do this already). From there build your products for different gift card amounts and then have people redeem the order in store. Most POS f&B systems I assume will let you keep a note for order customization's. In that field I would enter the order code after verifying that the order hasn't been marked delivered in Shopify.

The note keeping isn't super necessary but it can be helpful when trying to track down the order being given to a customer.

In this I'm also assuming Shopify lets you mark orders as pending, delivered, etc. I've only used competitors to it so I'm not 100% sure.

Could you do something with Venmo plus a manual ledger? Send money, write down the person's name etc., if they come in later you can look it up and give the credit.

The golf pro shop near me uses a laser printer and colorful paper. The actual liability is recorded in a ledger. You could do this with email and a spreadsheet.