Help please. Problems with digestive system and going to bathroom problems

Well how do i start, please stuck to the end. My girlfriend have some problems going to the bathroom. I mean she had like 3-4 control checks at the medics and they said she have no problems. She did a colonoscopy(that thing that check your colon) and still nothing.

The problem is,her body sometimes doesn't tell her to go the the bathroom and if she doesn't go by herself it leads to a problem that she won't go for the next 2-4 weeks. And when she goes after that much time not only that is painful but for a few days she feels bad and can't really do anything else because her stomach hurts. She take some kind of laxative pills..but it doesn't always work.

So my question is, how can i help her? I think she needs to respect a diet, not eating some sort of food. Also are there any exercises that can help her? Some tea or other kind of medicine? What would you recommend?