Is it normal that my gf joked with her friend about having sex with another guy

I have known about my GFs friend having a party for a couple weeks now tho she hadnt brought it up to me until recently.. after she found out id be leaving for California the following morning.I noticed her phone going off a few days ago while she wasnt around so I went to check it and to my surprise it was a text from her friend talking about my gf fucking another guy who is one of her "best friends". They dont talk much or hang out anymore since we both are extremely busy with work and taking care of my parents. In the texts my gf says how hes looking good and shes gonna hook up with him whether his girl is there or not.. Goes on to say how she was planning to go alone but has to hang with me since its my last night in town b4 my trip but that she will just have to sneak off. The next day they were still discussing it and since we had decided not to go due to a family emergency her friend commented "poor ***** wont get to hit that now." I didnt bring any of this up to her but it showed how upset I was and with my comments she knew Id seen but didnt say anything until it became to much. NOW she claims it was just a joke with her friend and that they always joke about this sort of thing because people never believe that he and her havent had sex even though they used to hang out constantly etc. I believe they havent but im pretty confused either way. My gf would never go out to a party or anywhere at all without me in the first place, we spend all our time out of work together or at least at the house together(We live in a very large house and can be there for hours without running into one another)