Blood clots from the upper legs sometimes dislodge and pass through the right side of the heart to become a pulmonary embolus in the lungs, effecting the segments of the lungs blocked by the clot which won't be able to oxygenate blood. They may want to insert a Greenfield Filter into the femoral vein, next to the groin, to thread it into the inferior vena cava so it blocks any clots that might dislodge.
They are probably waiting to make sure that she is hemodynamically stable, and that no internal bleeding is happening. If they want to do surgery, they won't anticoagulate, aka "thin", thin the blood. They also have to worry about kidney function with crush injuries. Prayers sent.
Thank you for explaining, sounds very scary! Thank you for the prayers.