A lot of people have suggested foods to eat to try and increase the fibre in your friends diet.
But I am going to offer an alternative explanation. Your friend may be intolerant to some foods. The most common intolerace to food comes from wheat and products that contain gluten, like bread. Food intolerance is not the same as a food allergy, but it means that your body is not able to process certain foods very easily. Resulting in bloating, gasiness, and constipation. Finding out which foods cause the intolerance can be a matter of trial and error. Try getting your friend to cut back on breads or similar foods first, as that is the most common culprit, and see if she finds her bowel movements a little more regular.