I would have to agree with Challenge Guy, because the part of me that says, "No way!", is that insecure part of me that's hampered me all these years. Logically speaking, of course, if she's known the guy for a year, she's not likely to all-of-a-sudden get with him, just cuz you're not there. If he's in The Friend Zone, it takes a lot more than just a trip somewhere (yes, even Hawaii) to get him out of there.
If for some reason she did cheat on you, it sucks, but at least you'd know after only 4 months than 4 years or even after marriage. Wouldn't you rather know now?
This situation is also tricky. Some guys on here would say that you let her go, you're letting her walk all over you. Well.... that all depends on you.
Response 1: "Well, honey. I guess. If that's what you want. Then I don't have any objections, and I'm sorry that I couldn't take you. (cuz that might be a BIG reason why you'd get jealous about it. Feels like this guy is one-upping you on your girl) I'm going to miss you a whole lot! Call me everyday tho, please?"
Response 2: "Wow, baby! That's cool! I hope you have a great time! Tell me all about it when you get back! Do you need a ride to the airport?"
Response 1 let's her know that you really don't want her to go, but you're letting her get away with whatever she wants.
Response 2 let's her know that you're not the jealous type and that you're happy for her and will support her.
Chicks seem to dig that. Wierd. ;-)