90 pounds for years

Hi, my wife has been about 90-95 pounds for most of her life and only has a bowel movement every 3 or 4 or 5 days... What comes out is dark, small, and hard matter. This has been going on for as long as she can remember.. She has a poor diet, mostly junk food, and eats a lot! She also has huge hunger cravings when it's been a few hours without food..
Anyways, any suggestions on getting her health back on track? I know she needs a 180 degree turn around, but what is most important to start with? Bowel?
Thanks for any help.