Your girlfriend should include all her income and expenses on Form Schedule C of the individual tax return Form 1040. The important thing to realise is that your girlfriend may be able to claim certain expenses that were directly incurred in connection with activities as a Mary Kay consultant.
But, remember, if the activity is deemed not be a serious activity, the IRS would deem this activity as a hobby activity and deny any losses generated from this business. Thus, I would be rather conservative with deducting excessive expenses, and the only expense I would deduct would be as follows;
1. Purchases of products that were sold.
2. Any meals and promotions expenses spent on inviting potential customers.
3. Marketing or Advertising expenses spent.
4. Business cards expenses.
5. Some portion of cell phone expenses (expenses incurred during the duration of the period your girlfriend was a Mary Kay Consultant).
6. Mileage generated whilst visiting customers.
7. Any licenses or fees paid to Mary Kay to become their independent consultant.
8. Any expenses spent directly related to Mary Kay activity.
Here are both instructions to Schedule C and an actual copy of the Form Schedule C. I am sure that you will gain additional insight into this form by clicking on the 2 links below. http://www.irs.gov/pub/irs-pdf/i1040sc.pdf
(Actual Form) http://www.irs.gov/pub/irs-pdf/f1040sc.pdf
(Instructions to Schedule C)