Ok, slow down.
Many women become pregnant and have a miscarriage without even knowing they are pregnant. A normal period just comes along and the girl never even knew.
You also could have had a faulty pregnancy test that gave her a false positive. This does happen also.
There is not a general reason for a miscarriage. Sometimes, the egg is not viable enough to survive pregnancy, sometimes the woman's body cannot carry the fertalized egg. But one rarely knows the reason for a miscarriage unless there are other medical conditions present.
Is it possible to get pregnant after this? Most definately. We don't even know if in fact she had a miscarriage. Like I stated above, it could have just been a false positive on the HPT and she never was pregnant in the first place.
What will the doctor generally do? As it was this early, if it was a miscarriage at all, probably nothing. There is nothing that can really be done. Later stage miscarriages require a procedure called a D&C, but not this early. Nature just takes over.
Do we need to stop having sex now? That is a question I cannot answer because I do not know that she was in fact pregnant. This would have to be answered by your doctor after a physical exam.
It sounds to me that she may have gotten a false positive and never has been pregnant, or that she is an "early pregnancy bleeder." Only your doctor can tell you this for sure.