My grilfriend[19f] has a family friend[18m] i am uncomfortable with

My girlfriend has a family friend named John, and she says that he's like a brother to her. When I started dating my girlfriend, I asked her where she was, and she told me she was at John’s house because both of their families had gone out. Then, I asked what she was doing, and she told me she was in a hot tub with John. This made me uncomfortable because I don't like my girlfriend being in a hot tub in a bathing suit with another guy.

Then, one time I was on FaceTime with my girlfriend, and she had just gotten out of the shower and was in her bedroom. At that time, I wasn’t allowed upstairs and not in her room. While we were on FaceTime, John barged into her room, jumped on her bed, and interrupted our conversation, talking over me. Then, another instance occurred when I was on the phone with her while she was leaving school. John saw my girlfriend and started playfully pushing her into the men's bathroom while she was trying to leave. He also asked her to hang out, which I don’t really like because I don’t like my girlfriend being one-on-one with a guy.

Now, John has started randomly messaging my girlfriend about silly things, like how to ship a package or how to work a school website. I believe he just wants a reason to talk to her. While John was dating his girlfriend, he didn’t talk to my girlfriend much, but now he has started messaging her since breaking up with his girlfriend. Also, prior to this, we were all playing a game, and he was only looking at her, teasing her, and swearing at her in a joking manner. I don’t believe they’re really like family since I haven’t seen him at any serious family events like Christmas, Easter, or Thanksgiving or New Year’s since we’ve been dating, but apparently, they did before. I don’t want my girlfriend to respond to his messages.

What do you think about the situation? What should I do, and what should my girlfriend do?