Tldr: I went out to dinner with two new people I was befriending who then started badmouthing my best friend. I didn't join in, but I didn't say anything in response because I was stuck with them for the rest of the night and didn't want to cause problems.

So, this happened years ago. I was leaving work and saw these two girls (I always refer to them as hitchhikers) sitting outside in the cold and I offered them a ride home. On the ride we ended up all chatting and we all kinda got along so we ended up getting food and hanging out a handful of times over the course of a couple weeks. Neither of them had vehicles so I always did the driving. Well one day we went to dinner and I don't remember how it came up, but shortly after we ordered my bestfriend was brought up and the hitchhikers just started talking really bad about her and saying some pretty rude stuff. I didn't join in or say she was my best friend. I didn't want to say anything about it because we still had food on the way and I was there ride home. I was stuck with them so I didn't want it to be awkward. I just waited for the subject to change and got on with the night. 
After dinner I asked my bestfriend if she knew the hitchhikers and mentioned it to her.
She asked what I did and I said nothing and explained. She got really offended that I didn't defend her at all and felt 100% betrayed. That was the end of our friendship. I kept asking her if we could talk about it but she just kept saying there was nothing to talk about.
AITA?