I know this question sounds stupid, but are you sure that is what she would want? Pain is an important part of life, as unnecessary as it may seem. It helps us to learn lessons in life and remember them. Like burning your hand on a hot stove as a child. You will always remember through life that it is not good to place your hand on a hot stove. Heartache is similar in that it reminds you of what went wrong.
If she is suffering tremendously, you could write a simple spell for her that would help her to move past the pain. Reminding her that she is loved and cared for will usually help. Also she could carry or wear a rose quartz, which main property is healing and balance of the heart chakra.
If it is causing her stress, a bath with lavender oil can help to ease the tension and frustration. This would also be a good time to do some sisterly bonding. Find something that you both enjoy that would help to take her mind off of it. Playing a game, watching a movie, dancing around to happy music, going for a walk outside or any other activity that does not involve said ex. Love, light and blessings :)