I would highly recommend the use of fiber capsules, pills or drinks on a regular basis, up to 4 capsules twice a day, if necessary. Continue the stool softener as well. The problem may also lie in too little fluid intake. Try keeping a large glass of water nearby at all times and drinking a bit whenever you notice the glass, even if not thirsty. This will help both medications to work better. The fiber is a mechanical way to bulk up her stool but she MUST have an increased fluid intake for it to work properly. It usually helps in cases of diarrhea as well as constipation and returns things to a more normal state. If none of this helps, she needs to be seen by either her regular physician or the surgeon to discuss her problems as there may be some problem with the 'j pouch' itself. It should only take a few days on the fiber and stool softener to see if things settle down. The Milk of Magnesia should not be taken during this time. The body naturally builds up liquid stool behind a plug of hard stool in an attempt to push it through so the MOM may be aggrevating that situation.
I hope this information is helpful. I am here should you have any more questions.