Seeing that the guy records it on camera - her girlfriend tries even harder. In addition, she wants to look even prettier - she fixes her hair, makes eyes, smiles. Knowing that the guy will show this video to his friends, she wants to impress them as best she can. Female logic!
All sisters are like that - at first no, no, but after a sleepless night they get their own dick in their pants. They just need to be given time to fantasize - with their wet pussy alone.