The DM allowed the NPC to be seduced. It's been said a million times but clearly that DM hasn't figured it out yet, the Persuasion skill is not mind control. Tell the bard "she just isn't interested and your continued attempts to woo her are just making the poor woman despise you more and more every time. In fact, you think you might have heard her muttering something under her breath about taking her chances with the big guy in the spiky black armor. Roll perception. She appears to have slipped away while you weren't looking."

If the bard decided to force himself on the girl that would open a whole can of worms but even if the table is okay with such behavior you've now pissed off the girl, the local lord, and the BBEG who (as others have noted) could probably find another suitable subject for his ritual with a modicum of effort. After he finishes beating the crap out of the bard.

DM should adapt... like, okay we'll summon another demon, just less powerful