Focus on your boyfriend
You don't trust her. Fair enough. Trust your boyfriend instead, it's much safer.
Talk to him to find solutions so you feel better during his trip (regular calls, information about this girl)
Don't make him cancel his plans because you don't feel safe about it. It's not healthy and it will cost you.
What I thought as lack of context, actually might be showing something from your perspective on this issue.
You made a lot of references of her and "them", and quite few about him.
Your opinion on her may be pretty oriented. The fact that she was his first love, and "she is really attractive, said other guys" could be a red flag.
It might also be you finding every bit of possible reasons having her in your radar and comfort yourself doing so.
I assume you already were together when this sinister_threat_name_here (@Stian Yttervik I couldn't dare change that name)
stayed over in his room. As you are still together, I also assume everything ended up okay!
You said two times you don't trust her, but never that you trust him even if "He has never shown unfaithfulness at the time or ever really".
It seems to me that you see him as a guy without any will that could be turned at any moment if that girl just decide anything.
However, he is with YOU. And that should be it, but it's never that simple right?
My ex use to be very attractive (to me, and other guys. Or more likely, I use to think every guy on earth found her attractive!)
She had a lot of dear friends, including single guys, including one who formerly was in love with her.
Spoiler alert, she isn't my ex now because she is with one of them, but one of the reasons probaby is the trust problem I had with her!
Now i'm not saying you are wrong and shouldn't feel that way.
But it doesn't really matter that you don't trust her, as you should trust your partner instead, and it should be much more conforting and important.
Still, as you are not comfortable with this situation, and you are in a serious relationship, both of you need to find a solution to this problem.
Let's try to enforce that trust. Try to identify your fears, and talk to him about it in a safe space, so you can communicate about it.
This wouldn't be about this girl, but about your relation in general. Is there something lacking from your perspective? What is it? He needs to know.
Discuss with him about anything that could make you feel better during the trip.
Him calling you on regular basis, maybe you getting a first contact with this girl? Asking about their current relation?
You might see her as a threat, but it's fairly possible to be very wrong. Do you have all the information about this girl?
She might like someone else or they might be 100% friend with nothing else. Your boyfriend want to visit her, it's okay to be curious and try to understand in what state she currently is. It might be so obvious for him, so he doesn't even realize there is an issue for you here.
You can find some comfort doing so.
What could also help :
Ask your boyfriend to call her when you are around.
During the discussion, he would then mention naturally that you are here with him at this moment.
If she hears your voices or knows your presence on the back, you become real.
They can then discuss about the trip, and you could naturally be included in the discussions.
Even if this is doable, I wouldn't advise you to convince him not to go.
For couples and friends, it takes a long time to build some "caring-capital", and such action has a great cost of it.
He needs to decide by himself, and also, makes sure his girlfriend feels good and secure with this.
After all, he could very much face a situation alike, later on, and would be glad to see kindness and understanding from your side!