My daughter was about 18 mos when I got pregnant with my second and she was still nursing. I didn't do anything to wean her from the start but when I was about 4 or 5 mos pregnant, my milk was gone. She continued to nurse and it was actually hurting me so after talking to her Ped. at her two year old check up, he suggested that I just talk to her. That she will understand more than I realize. That appt. was on a Monday. For the next few days, I told her Mommies milk is all gone, it hurts Mommie's boobies to nurse. Thursday night of that week she didn't even ask, she patted my boobs and said, "milk all gone" it was the sweetest thing. With my second, I night weaned him around 2 years old and I did this by telling him that when Mr. Moon is out we don't nurse, he could nurse when Mr. Sun is out. This took weeks but it did eventually click. So my suggestion (sorry, long story longer) is to tell her to talk to her daughter. If she is trying to wean her completely tell her to keep the dialogue simple and keep things positive. My son who is almost 3 (I never thought he'd nurse past 1) nurses about every 3 days or so, not much at all. Well, I've been mentioning to him that when he turns 3, no more nursing and so far he has agreed to that. He'll be 3 in April, so I've planted the seed early in hopes that when the time comes, he won't have a melt down.
Best wishes to your daughter,
M.