If you let someone else use that beef offer you will just get it more frequently, and in fact you will make someone else's grocery patterns take over your card so at this point what's the point of having a personalized card? If you don't like an offer just delete it, get a better offer, and move on. Better yet figure out why you got the beef offer and buy the same thing again.
If you want someone to shop and earn points for you and never redeem them, set up your grandma with a card.
No program is meant for strangers to collaborate on collecting points, not even for charity, so I think this scheme is crossing a line. It's one thing to set up a scheme to share points with a roommate...it's totally something different here. Do no business with strangers on points cards - it is expressly verboten! I would never want to see an rfd'er in trouble. I mean this in the friendliest of ways.