Hot video, but this guy has absolutely no idea what he's doing, or he doesn't care. She is not enjoying the anal at all. Use some lube. Lots of lube. Prepare her for the anal - give her an orgasm or two first, relax her anus with gentle touching and pressure, or better yet, eat her ass until it's relaxed. Only then, start to penetrate her ass, slowly, and hold still until she is used to the feeling, and you feel her relax. Rub her clit at the same time. Then you can begin slow, gentle thrusting. Listen to her and speed up as she gets into it. Give her an orgasm while she's receiving anal, and she will want it again.
This poor girl: this experience probably turned her off to doing anal in her private life. Just like a lot of women who's initial anal experiences were similarly bad.