You should do more searching on Nolva -- it doesn't eliminate existing fat but will help prevent additional body fat depositing as part of the regular estrogen cycle. Note that if your girl is on birth control the nolva will effectively neutralize any pregnancy protection -- when I considered doing this for competition prep while on b/c, my doctor said why not just stop taking the b/c?
There is not much in the way of research regarding nolva use in pre-menopausal women - most is with regard to post-menopausal women in the context of breast cancert treatment.
I suggest that you will get better results with fewer side issues (e.g. birth control) using ECA. At age 17, your g/f should have fewer issues with weight and lack of metabolism response than when she is older. Take advantage of that before getting her involved in "alternative" methods. Better long-term results will come from a clean diet & training lifestyle.
But for your knowledge and hers, please use the search feature here and spend some time looking on the women's board -- "supplements" used by men do not necessarily have the same results and definitely not the same sides as for women.