Instead of explaining to a waiter or waitress that your dinner partner might have trouble understanding the menu and ordering, how about this?

Hand a business-size card to the waiter or waitress that says: Please be patient with our order. The person with me has memory problems. Thanks for your understanding.

This way, the waiter or waitress understands, and probably will be more patient. At the same time, you won't embarrass your dinner companion by explaining the situation out loud.

You can buy very inexpensive heavy blank business card paper at any office supply store. Print just one sheet out on your home computer, break them apart, and you will have a dozen or more discreet cards to carry in your pocket or wallet. Just one package of these blank cards can last practically forever. 

This suggestion from ActivCare at Mesa and Point Loma, which has these cards printed out for visiting caregivers.