I'd definitely recommend getting a credit report (not from the websites that advertise with an insane jingle, but from the actual credit bureaus

you're entitled to a free report). Mine had debt from a relative with a similar name; I was able to get that removed. They will also tell you in more detail what goes in to calculating it.

I agree that it's not perfect, and often very opaque, but you should be able to get some understanding of why she doesn't have good credit.