All the 5* hotels sell vouchers and you can use them anywhere in the hotel -the spa, dining etc. They might have some restrictions on them, such as times or days, but for the most part, no. You would just buy a voucher from Jumeira, for example, as Catawba said, for say 200 dirhams. The only thing I would make sure to do is, when your friend calls to make a reservation using the voucher, they have to tell the hotel / restaurant that they are using a voucher. It's for the hotel's records.
You can buy one from Jumeirah hotels that can be used in any restaurant in the group in Dubai which would give your friend great choice. Not only is it good at any restaurant but also for hotel stays and spa services across the group. I just bought one from the gift shop in the Emirates Towers hotel and have a friend who bought one at Al Qasr gift shop, so assume they have at all gift shops. Make sure you get the right one if you want it for across all hotels as originally they tried to sell one just for the individual hotel; initial person helping me didn't know they had a group wide one.
I havent heard of any restaurants that do this, be good to know thou. Maybe you can pre - book a table with a said amount, and make a voucher up yourself.
Does anyone know of any fancy/nice restaurants in Dubai that sell gift vouchers so that I can give it to a friend so they can enjoy a nice meal out? (if that makes sense!?)