How can someone have an AB+ blood type have an Rh- factor?

AB type and Rh type are not related.  They are caused by different sugars on different molecules.

The '+' or '-' refers to the Rh factor, so not really (the ABO type isn't really relevant). Someone who is Rhesus D positive has the rhesus antigen on their blood cells and doesn't have the antibody against that antigen (otherwise they would attack their own blood). Negative types are the other way round. This is why negative types should generally only receive negative red blood cells because they have antibodies against the positive types. (Positive types can receive either.)

Are you sure she's not just AB type but Rhesus minus?