It can lower platelet counts and white blood cell counts (can even be associated with aplastic anemia) but these are very rare occurrences. Much more common is the disease itself=lupus causing these blood abnormalities.
There are other reasons as well, which is why they are doing the bone marrow biopsy too. Write back to let us know what you find out, and best wishes for a good outcome.
As Maia said, it's probably most likely lupus is the cause. The marrow test will show if she's producing platlets. I had Platlet trouble a few years before I was diagnosed. Marrow test showed I was making the platlets, so it was assumed my spleen was filtering them out. I was put on 100mg of prednisone, my first experience with pred., a drug I now know VERY well! Pred. did the job and I've never had a return of that particular problem. If the Pred. hadn't of worked the spleen was comming out!
In my case my count was very low, around 1,000. I had a horrible time with brusies. Poked my arm with a screwdriver and my whole arm turned black and blue!