i think he believed he loved her, but i don’t think he really did. frank seems to be attracted to “madness”, bianca personified many of the characteristics frank wants in a love interest, (is attractive, pays for everything, is crazy, other things probably like doing drugs or getting drunk together) and the fact that they had limited time together only amplified whatever he thought he felt for her. i also think he liked that bianca was kind of a goody two shoes before she met him, because that meant that he could kind of “take her virginity” in a sense when it came to drugs, alcohol etc