Hard and fast rules with Chinese herb/western medication interactions are hard to come by. Even where there is a complementary or inhibitory reaction, this doesn't always mean you wouldn't use a particular formula. Part of this is simply because it is dose and case dependent.
Femara (letrazole) is an aromatase inhibitor to reduce estrogen in the body. It can be used for general cases of hormonal imbalance, particularly with estrogen dependent cancer histories (or activity) - but also for fertility cases, for PCOS symptoms, etc.
The real question then is in this case what is letrazole being used for and what is the intended duration? On top of that, why shi quan da bu wan? What is the patients TCM diagnosis and what is the intended outcome of the herbal formula and at what dose and what duration?
With that information, input can be offered. In many cases letrazole is being used to treat conditions that generally would do better with either acupuncture and/or Chinese herbal medicine alone. If it is to balance side effects of letrazole, then there are a number of studies showing the effectiveness of acupuncture for this ( 1 , 2 as examples).