need advice

My husband is in the army and stationed in Korea. He's an NCO. Today we were on FaceTime and he showed me that he received a (Father's Day) greeting card from a female soldier (soldier who's a rank under him that's a specialist) the front of the envelope said "It takes a man to become a father, but it's take a person to be a dad." Then I asked him to show me what the card read and it was handwritten and while I don't remember everything it said, it was something about that she's proud of him and that he's a good role model for his soldiers and that his family is lucky to have him.  And on the right it said, "To SGT ••••••• from SPC •••••." And I didn't want to read what the front of the card said because I jumped to wanting to read what she hand wrote. 
I feel betrayed and been crying about this. I'm waiting for him to wake up and tell him how I feel. When he first showed me, I was asking him why would she even send that? She has no reason to send that to him at all. She has no relationship with him. She is his wife, or relative. And I find this to be very inappropriate of her to be sending any greeting card to a married NCO. I feel like I should report this to their commander. And that will certainly get her in trouble. I feel so sick to my stomach and that is not from my pregnancy! My husband is a very responsible man. I don't believe he would try and jeopardize his career over this woman, or any woman for that matter. I think she has a big crush on him! I have her now on FB since a few weeks ago when she messaged me to tell me I'm so beautiful and so awesome and that it was so nice meeting me when I was in Korea. 
We announced our pregnancy on Thursday and she even "loved" the post and said we are a beautiful family. I can't help but feel that she's jealous and she's trying to come between us. She has only been in the Army for a year, so I think she has no idea of her actions that could later hurt her.
I need your advice. If you were in my shoes, what would you say? I almost feel like going to Korea for a week to spy on him. Or hire a P.I. Do you guys think she was out of line for this? I've expressed with my husband before how she makes me feel uncomfortable and he said that I have nothing to worry about, that she's not that kind of girl. I'm not there, I don't know! My husband is not the cheater kind, but he's human. And plus she's not ugly. I'm really worried, feeling betrayed, crying, upset and disgusted. What would you do? What do I do? What do I tell her?  It bothers me that she knows where his room in the barracks is so I asked him how does she even know? And he said that the whole company is in the same barracks so she probably looked it up. That bothers me!!!
She was out of line twice!!!