I have been in your friend's shoes. There is no quick answer except to not stress over it. The bigger deal that Mom makes about trying and accidents, the more stressed the child will be and the more she will hold it in.
My daughter can go sometimes 12h between bathroom breaks.
Here is what helped us.
1. Get a soft chair that is low to the ground and easy to clean. Get one for each bathroom, or at least each level of your house.
This is the one we used.
http://www.walmart.com/ip/Safety-1st-Comfy-Cushy-3-in-1-P...
This one is similar and Princess and has a side handle.
http://www.walmart.com/catalog/product.do?product_id=1425...
2. The DVD Potty Power was a BIG hit and help!
http://www.amazon.com/s/ref=nb_sb_ss_i_1_11?url=search-al...
3. Any time she used the potty and had any bit of success, she was rewarded with M&Ms and high-fives.
4. Any time there was a mess in the pants, there were stern words, but no spanking. Spanking is NOT helpful when potty trainging. We went upstairs, (at a snails pace) to the bathroom, even if my kid was yelling to hurry up because she felt icky. 'well, you should think about that before messing your pants. you knew you had to go, nothing was stopping you from getting to the potty.' There she took a shower. Put on fresh clothes, in the bathroom. This got old real fast for both of us, which also helped her understand. I kept about four pairs of clean underwear in each bathroom.
When my son first started training, he was afraid of the dark bathroom, so I put the potty in the middle of the kitchen floor! That worked. Then after a few days, I moved it to the next room, the laundry room. That was fine. Then we finally moved it to the doorway of the bathroom, where we kept the lights on all day so it wasn't dark. That finally did the trick as to keeping it in the bathroom.
As for the constant constipation, has she talked to her dr. about using Miralax? For us, we were told to give her 1/2 tsp a day. We give it to her in either orange juice, chocolate milk, or strawberry milk. It does not work right away. It sometimes takes 2 -3 days to soften things up in there. During her really bad spell that included an ER visit because she held it for 6 days, we gave it to her for a whole month straight. Even if she was using the bathroom regularly.
She can also try introducing probiotics like Activia yogurt. That works well too on constipation.