What are you after from this woman, a relationship or sexy fun?
If it's sexy fun then it's easy just say you're not after a relationship either but just after a bit of fun.
If you are after a relationship with her then you can't really force her to be in one so take it on the chin and don't have any ill feelings towards her stay friends be a bit flirty and who knows. Also go out, approach and meet other women, don't get hung up on one.