Was this a sick joke or do you think I shouldn't think about it anymore?

Okay so my BFF is the greatest girl in the world. She is the sweetest person you will ever meet and she never wants to hurt anyone. But lately (do to some confusion and an encounter with my brother) there has been tension between us. So about 3 weeks ago a got a call from an unknown number but I missed it so I called back. I was like 'I just got a call from this number, who is This?' and this guy goes 'it's (guys name)' and I was like I don't know you and he goes '(bff's name) told me you like me and wanna go out with me, how 'bout it?' and I was like 'no, that's a lie. I don't like you cuz I don't know you and (bff's name) would never do that. So good bye and dont call me again.' Well over the weekend I went to her Halloween party and her cousin was there. Well her cousin is mentally retarded so he takes things seriously (know what I'm saying?). Anyway we were all playing hide and go seek tag in the dark cuz her backyard is huge (we were bored and their were tons of us) so anyway me and her cousin were it together for one of the rounds and he asked me if I got his call and I was like no and he was like 'haha yeah you diiiid I remember talking to yoooou! I'm (his name), rememberrrrr?' (bingo! Same name, bff's cousin!) I knew the guy on the phone was him and I was in shock because my BFF really was the one who lied to him! So I pulled her aside and confronted her about it and she was acting like it was no big deal and that I could just laugh him off and say no. But it wasn't that simple, remember this guy is mentally retarded so he took it literally. In my opinion I think it was a sick joke on him because he thought she was serious. So what I wanna know is if you guys think it was sick to use her cousin like that to play a joke on me? Thanks!