So there are a lot of reason that led me to believe that she did actually cheat.
The biggest giveaway for me was when after he told her that it was a joke and there was a camera, she turned around in disbelief and softenly asked "what?" and right after covered her face with her hands which is known as blocking and symbolizes shame.
Right after that scene, he asked her "are you actually serious?" through which she did not explicitly reply because if she said "no" he wouldn't beleive her due to her body language, and if she said "yes" it would have been all over and 100% confirmed. So what does she do? She just "oh my god" because she cannot believe that she got busted and is now in this huge mess, about to lose her bf.
Also, if she was acting, she would have told him it was a joke before he left the room and wouldn't have pushed it as far as she had. So why didn't she? Because in my opinion it was easier for her to admit it was a joke in a different and more relaxed setting than in the heated room they were both in, and it also gave her some time to regroup and then actually pretend she knew it all along.
Then there are all those mini signs of her touching her neck so much, her hair, the guilty and sorrow eyes, and in general I felt like the whole act was too genuine and real to be acted, esp with all those hand movements of touching him. To me this means that she still loves him and wants to be with him and that's something most people would prob do if they would want their partner back in the moment. If she knew it was all a joke from the beginning she wouldn't have tried to touch him as much as she had and would he stopped when he told her to.
That's just my opinion though, I may be wrong.