I totally understand your concerns but it is true what they told her - 9 out of 10 times it is nothing, just benign tissue.
But, of course, there are other issues that are important to know. For example, if she has family history of breast cancer (mom or sisters), she is definitely at a higher risk for having it as well. However, given the fact that it was found on mammogram, even if it is bad, it is very early and having surgery will get rid of the entire cancer area. What I am trying to say that even if it "bad", it is still ok because it was found early and is curative.
One of my nurses at work (with history of breast cancer in her mom and in her sister) had a spot biopsied which turned out to be early (very early) breast cancer. She opted to have both of her breasts removed (because of such heavy family history). There was actually cancer in the other breast too but too small to detect even with mammogram. So she is 100% fine, had breasts rebuilt and is doing great.
Your friend should be able to do a biopsy within the next 4 weeks. Hysterectomy should not interfere with getting it done after the first week or two.
Don't worry, chances are it is nothing!!
Sorry for such a long dragged out answer :)