Chances are, the miscarriage was a given.
Nevertheless, that in no way excuses by any means the store from letting her go get medical treatment. Aside from the fact that she could have ended up with uncontrollable bleeding, that is embarrassing for any woman and traumatic for a pregnant woman to go through. Any woman being pregnant knows that if you start bleeding during the pregnancy, you get medical attention ASAP.
My wife started spotting blood during both pregnancies. She miscarried the first time but delivered a healthy, future Marine the next time.