Can we sue someone for stealing our business?

My wife and I started an online business and it exploded a few months ago. Her "friend" found out we were making money and is copying all of our best selling products. She is copied all of our products and posted them on the same website and undercuting us by $10. Every time we post a new product, she copies it, and makes it cheaper. It''s as if she''s trying to destroy us. Our profit has been cut in half. It is getting very frustrating. My wife is having a nervous breakdown. She is so stressed, she lost over 15 in the past few weeks. The lady has been obsessed with my wife every since they met copying everything she does. She moved into the same apartment, copied her appearance, even got the same eyelash extension. Until now, they were good friends. She came over all the time and played with our children. Now this. Is there anything we can do legally to make her stop? We don''t care that she is using the same website. We just want her to stop selling the same products and copying us.