How would you handle this?

DD (almost 4) developed a friendship with a girl at preschool who was born without a hand. DD really struggled in the fall with socializing and making friends and when this girl came to her class in January, they really hit it off. At first, DD did ask me "what if I had just one hand?" and why does the other girl have just one hand. I reassured DD that she was always going to have two hands (I think she thought it could happen to her) and that God makes all of us different and that this otehr girl can do nearly everything that DD can do, but that she just might need some help sometimes. After this conversation, DD would talk about playing with this girl all the time and how much she liked her, but never brought up her not having a hand again. Then randomly while we were at home, DD finished drinking from a cup and put the empty cup over her own hand and said "Look Mom, I have no hand!" I told her not to do that and she took the cup off, of course aksing "why". I just told her it wasn't nice. I was worried that she'd do it in front of the other girl and that the other girl would be hurt. She's done this now about 4 times at home.  I just read a book that talked about kinds role-playing as a way to empathize with someone else's situation. Should I let her do it? DD really seems to like this girl and I'd love to have her over for play date and cultivate a friendship, but I'd be mortified if she did this infront of the child or the parent. Any advice? Should I mention this to the mom before I have her over for a play-date?