Trips are often very hard on friendships, it's not just you guys. People's quirks come out that are annoying to other people, and the close contact can trigger. That you don't remember what you said is kind of awkward but I can relate. Maybe a couple's counseling type session with your friend, or the cousin, or even your boyfriend, to discuss what happened. That way the conversation would be more directed than just a lunch. Kind of like when they have peer moderators when kids get into arguments at school?

My mom was ready to kill her brother-in-law after a trip to Florida for how she perceived he treated her sister, and she actually did lose a friend she went on a short cruise with. She blames both problems on me, of course, for encouraging her to take these trips, but I think the common denominator might be more her than me...