WHEN your daughter is ready (meaning when she is absolutely certain her son is ready to take control of pooping in the potty) have HIM take control of it. My son was nearly 5!!! I was so frustrated because he never peed in his pants, but always pooped. It made me so mad. Finally I told him he was responsible for all the cleanup. I bought boxes of babywipes, saved my grocery sacks, and got mentally ready.
Everytime he pooped his underwear (because I refused to buy pullups for him anymore), I went to the bathroom with him and told him gently what he needed to do i.e. take off your pants and check to see if there's any poop in them. No? Okay, now take off your underwear carefully. Now drop the poop in the potty. Now hold on tight to your underpants and put them in the water while you flush it (yes, I made him do this, but the parent may need to do this part)...
Here's the kicker... poop would often get down his legs. He was to take the wipes and wipe every speck of poop on his bum and down his legs, and on the floor if any got there. The dirty wipes when in a grocery bag which he had to throw away outside. I KEPT MY NICE VOICE and didn't get mad at him at all. THEN WE ALL WASHED OUR HANDS.
It took about 5 days until finally he said, "Mom, I don't like doing this, it's gross!" LIGHTBULB. So I agreed wholeheartedly and said, "Yes, son. If you would do your poops in the potty it would be much easier for you wouldn't it?" I don't think I had another problem after that :D
So for what it's worth, tell your daughter good luck with however she chooses to do this!