So you mentioned something along the lines of you don't know what to say or what she might want to hear? You and her should both be able to sit down and talk about the entire issue completely without being afraid of each other's reaction. You personally should also probably meet with this guy at some point, It's a somewhat unsettling thing to just kind of wonder who the guy your sweetie is meeting up with is like. Meeting the guy, talking to, and getting to know him will help you be okay with their potential emotional connection in a bigger way than you might think.
Another big thing to focus on is NOT focusing on their relationship. A curious "how are you and so-and-so?" here or there is fine but stop yourself if you find yourself wondering about every interaction they have. If you and her care about each other as you say you do then focus on your own beautiful relationship, and be happy for her being happy. Take joy in whatever makes her happy (unless it's something particularly damaging to her that is) and remember that just because he makes her happy doesn't mean you don't make her happy.
On the same note of not focusing on their relationship too much, and this is just a personal rule of mine that I have found helpful, never ever ever ever attempt to quantify love to yourself or anyone else. Don't ask yourself or her if she loves him more than you or anything of that nature. Try not to allow yourself to wonder that either because the simple answer is she doesn't know and neither does he, or you for that matter. Love is nearly impossible to quantify and if you attempt to quantify it then you will only get problems out of it. In short don't focus on how much anybody loves each other, focus simply on that she loves you and you love her. Focus on the fact that the love exists, not on how much of it there is.
I'm not sure if this helps or even hits on whatever issue you might be having but I hope things go well for you anyway. I kind of got carried away but you should start with being completely unafraid of each other's reactions.