As you might already know, placenta is the structure which attach the fetus to the mother's womb. Any breach to this attachment can cause bleeding from the area. A sub chorionic hematoma
is the collection of blood caused by such mishaps. The out come is largely depends on how big the bleeding - that's hematoma is. Smaller ones usually do not make much of problems and get absorbed and the breach is healed by normal regeneration. Larger ones can be dangerous, both to baby and mother. So it is important to prevent further progress of the bleed. That's why her doctors have advised limiting physical activities and penetrative sex which can provoke further bleed. Many doctors try uterine relaxing drugs, hormones etc but all are of unproven results. What she need now is rest, plenty of fluids and follow up. Wishing her best.