When I was potty training my daughter, my mom warned me that:
Often times your child will regress after being potty trained
And what do you know....my mom was right :) When my daughter started having accidents again I had to come up with a new plan of attack. That's when I introduced drawing faces. Each time she'd go the the bathroom in the toilet, she got to draw a face on a piece of paper that I had hung up on the fridge. When my husband got home, she'd get to show him her beautiful artwork. This little change of reward seemed to do the trick.
I currently have a friend dealing with this situation with her daughter. What are some tips you'd have for her?