Everyone has said some really great things here. I think it could be very beneficial to talk to the guy she is going to visit. He needs to know you are real and that he is not the center of her world. You are. Your kids are. There is nothing more of a reality check than meeting a primary and the kids. It brings that NRE to a grinding halt and everything becomes desperately clear for that time... I'm sure he and she will catch on quickly as to if their relationship is worth pursuing when they add you to the mix in their communication...
I suggest also that he come to the two of you... you could offer to pay for some or even all of his airfare, just for the pleasure of his company. I really think that this would add the additional reality check for them and help put it all into perspective. He could stay at a hotel, or you could set him up at a friends place and then they could have dates everyday and take it at the pace it should go at.... which it SLOWLY!
If it were me, I would be very firm with this and explain that the way they are planning is putting your marriage in jeopardy and that is unacceptable for you. You are part of this arrangement, whether they like it or not as YOU are the primary (to me if there are children involved, there needs to be a primary couple). You will have to stand firm and remember that the pace should be based on how you feel, not them. It's okay to back down and say, "actually sweetie, I'm not okay with you going to him... I would feel better if you stay here and give it more time or have him come to us." It's okay to change your mind...
keep talking, keep reading, keep asking questions and STAY STRONG. She is in lala land with her new love (NRE-New Relationship Energy), she is not necessarily going to make the best decisions for her family, she is not necessarily going to think of your best emotional interest, she is not necessarily going to think of anything but herself.... you need to do that for her and keep her in check with it all. At the same time you will have to be willing to give, but not at the expense of your mental health and to the detriment of your family. You come first and so do they.... she might of forgotten that.
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