Welcome to the forum.
The other guy did a shitty thing. But - he was speaking truthfully about his limitations: he thought he could share her, but doesn't want to in reality. So he got that out there. Either this was his plan and motive all along, or he developed that feeling after some time with her.
In my mind, it's actually your girlfriend that has made the wrong decision here. Absolutely no offense is intended here - I am just giving my opinion. Why is your girlfriend still seeing him, knowing his stance on things?
I do have a little experience and some thoughts for you.
I don't know how many prior relationships your girlfriend has had, but I definitely think that the 20s are an exploration decade. I first entered into a poly agreement when I was about 23 and it was a similar situation to yours. I'd been with my then-boyfriend for about 4 years and developed a strong crush on a girl at work. He encouraged me to pursue it, as long as he was the one I was coming home to.
I felt that I could get girlie romance, talking and amazing sex with her. With him, I had a deep emotional connection. I fell in love with her and thought that I had to choose one person. I didn't know that it was possible, or acceptable, to meet different needs with different people.
But, as others have said - poly means that we do not necessarily have to choose. I do agree that polyamory is *technically* about loving more than one person. I also agree that, in the end, it is naive for many of us to try to prevent this from happening (for ourselves, or our partners). However, it is also my personal belief that the way poly is balanced has a great deal to do with what our needs are and what priority each need takes. We don't have to fall in love. We don't have to have a ton of 'primaries'. We don't have to have any hierarchy at all. We can, if we want - but more importantly: we can if it *makes sense*.
For example, when I met that girl I had a crush on, in my early twenties, I placed a high importance on sex, exploring my attraction to women and socialising. I needed those things at the time. I spent more time on those things during that poly relationship - I spent more time pursuing the girl than paying the right amount of attention to my existing relationship, because he did not provide as much of what I thought I needed at the time.
I could have continued, calling him my primary and her my secondary. But she was fulfilling my 'primary needs'. So, how would this make sense? How would this work in the long term?
These days, my primary needs have changed. I find almost complete fulfillment of these new needs with my girlfriend - therefore, I consider her my primary. I still have other needs - but these are secondary in importance; so, I fulfill them with secondary partners. I spend more time and energy on my primary relationship, because it fulfills my primary needs. Does that make sense?
In the case of your girlfriend, she's 26 and discovering/exploring what her needs are. She needs romance - something you've both addressed. This must be quite an important need for her. What if romance isn't a secondary need - what if it's a primary need that you simply cannot completely fulfill? The same thing regarding sex. What if that is a primary need for her? If the sex never ends up being fulfilling with the other guy, I doubt she would be happy only with him, regardless of the romance.
What if she could have all of her primary needs met by you and this other guy? What if she was open to having both you and him as 'primary' partners? What if she still thought of you as a primary and found the most fulfillment in you, but was still open to falling in love with him?
I think those are the important questions to ask. Once you've both thought about them, you will know what to do in terms of her dating him.
But, regardless of anything else, she needs to decide whether she definitely wants to remain in a relationship with you.... If she does, how can it work with this other guy? Is he going to be willing to share nicely?
Good luck with your talks - I will check back and see what your updates are