My fiance was using the restroom and noticed some blood in the stool. This has happened in the past. What could it be?

Many things. Get it checked; could be simple as hemorrhoids or fissure or as dangerous as cancer, depending on age, family history, etc. If the blood is bright red, the cause may be evident on a rectal exam or sigmoidoscopy. Alternatively, a colonoscopy may be necessary. Either way, I do not ignore bleeding.
See doctor. Rectal bleeding is not normal and should always be evaluated. See a colorectal surgeon, who will do a complete evaluatin and examination, and can treat most causes of rectal bleeding.