How much shud u flatter or flirt with a gal who has a bf?

She has boyfriend, and flirting with her is already offensive but since she likes you flirting with her its not offensive anymore at all, i guess, the limit is only by not asking her to sleep with you but you can tell her that you are imagining her with you on the bed. She likes being seduced, she feels beautiful with this and confident, i guess..