This man makes me feel sooo uncomfortable

So last week my best friend had her birthday and she organized a trip to celebrate them. In that trip, we were 5 girls and 1 boy. I didn’t know him. We had booked two rooms and I would have to stay with my best friend – let’s call her A.- and him, let’s call him B. After spending some time all together, I and B decided to go on a walk, the rest of the group didn’t want to join. We had some fun, we bought some souvenirs, nothing too special. Then, it was time to find the rest of the group in the club we would spend the rest of our evening. And it was then that trouble occured. B. was touching me all the time, he was staring at me, pouring me drinks etc. I thought that I had given him the wrong message during the stroll. I kept changing places, dancing with the other girls but he was following anyway. I couldn’t do anything because I didn’t want to ruin my friend’s party. When I pretended I was bored, he would ask me what was wrong. Even when I went to the restroom to calm myself he came to check on me! Anyway, the worst are still ahead. When we left the party one of the girls was very drunk. A. said that she would sleep with her so I had to share the room with just him. I was terrified. I went to the room and I told him that I was too tired and that I would go straight to sleep. He insisted that I should come sit with him to have a chat. I said no and I locked myself in the bedroom. The creeper kept shouting my name and asked me to come out so we could have a chat. I was on the verge of crying. I texted A. finally telling her what I was going through the whole evening. I left the room and joined the other room. I was in shock, I was trying to tell the other girls what had happened only to receive comments like “Oh, he’s just into you!” Anyway, A. came with me as we had originally planned and we finally slept.
Fast forward to the present moment. A. is still probably oblivious of the fact that B. scared the s**t out of me, and believes that I’m overreacting. B. proposed to go out 4 people of the group to listen to a very popular singer that I really want to go. However, I don’t know if I should. I don’t want to spend another night with him. I don’t care if he is -according to A.- “the best guy you will probably meet”. What should I do? How can I set boundaries without having to create friction to my friendship with A.? And, is there any case that I have indeed overestimate the whole thing?