I had an issue with this with my first child and stopped breast feeding, with my second I was lucky enough to have a nurse who showed me what I was doing wrong.
The baby is not latching on right. The nipple has to go in centered. The easiest way to start breast feeding and get the right latch on is the football hold. There are a lot of pictures online you can find, but basically using your boppy or pillow lay the baby next to you, under your arm, with their feet going toward your back. Holding the back of the baby's head lift her up to your nipple. If it hurts, use your finger at the corner to gently break the suction and try again. Get your nipple in so that an equal amount of the aereola goes in. If it is off center when the baby comes off it will be more the shape of an orthadontic pacifier and that is when it hurts. Also the baby is not getting milk as easily this way.
If she tries this and it still hurts have her go back to the hospital where she delivered and ask one of the nurses to help her. There is usually at least one that is very good at this.
Once she and the baby become familiar with how to latch on, then she can change to other feeding positions and it will be easy, she just needs that learning time.
Good Luck.