I voted person A for these reasons:

Person A went to Person B's home willingly. Person A already experiments with substances and was already under a chemical influence (been around the block per say) Person B was (in my opinion) being a generous host by providing Person A with a complimentary recreation so to speak. Person A could have decided that the copulation was likely before taking the pill. Speculation only, motives are within the innermost confines of the chambers of our hearts, concealed. If person A expresses a level of adverse feelings about the encounter, then there is a problem. Person B should not feel guilty if all was consensual. Only if Person B pre-meditated taking advantage is there a fault. Person C sounds well meaning, but Person A and B are the main characters and this should be between them.