My Friend Has Been Diagnosed With Triple Negative Breast Cancer.

my friend has been diagnosed with triple negative breast cancer. The chemo is entered through a chest port. The chemo is cytoxan, paclitaxel, doxorubicin. She has had 2 chemo treatments so far. The lab test last week showed a 1.7 on her white blood count. Why is the Dr. continuing this treatment with these low levels of white blood count? when is it considered life threatening low?