I would be concerned about her as well from what you have told us. She seems to lack maturity. I know that she is recently qualified as a teacher in her 1st job which she has to find her feet in and deal with issues that come up.
If she gets 2 years experience in the UK it can help her get a proper work visa in the USA if that's what she wants to do and does not have a boyfriend rushing into this.
The reality is that she with a man who has more life experience than her. He is pushing her towards moving to the USA for his own reasons and telling her he will mind her.
If he is 29 with a good military job why is he not all ready married? I wonder did his ex girlfriends and her parents notice his control tendancies.
I tell her that at the moment the only visa she will get into the USA is a 90 day tourist visa and she can't work on this visa. She will have to produce her passport, this visa and tell the USA boarder officer where she is going when she is in the USA and show them a return ticket.
Tell her they can and do ask people questions despite having this and they can stop her entering the USA if they feel she is going to stay their after her 90 day visa is up.
Getting a visa into the USA where you can live and work permanently on it far from easy and it has a large amount of paperwork and expense. She will have several meetings in the US embassy for this and will be questioned in depth as well.
I ask her does she want to work in a school where they have regular drills on how to deal with the situation if a mass shooter gets into the building.
At this stage I get your husband and any other person she trusts around and tell them you have decided to set the boyfriend up and you need their help in getting him out of her life.
Tell them why don't we bring him around to the house for a family meal and start to bring up things and ask him loads of questions.
Let's ask him the following:
Ask him how much does he earn? Tell him you want to see his bank statements.
Ask him why told X that you don't want her to see her friends since she met you?
Tell him that you have heard that married quarters far nicer than a dorm room he has currently as a single man in the army.
Ask him how is he going to support her if she moves their. Ask him how much is her medical insurance going to cost him a year? Ask him what visa she needs to move to the USA with him?
My feeling is that he thinks she move over with him and has no idea that it's going to be far harder and more expensive than either of them realise.
Ask him does your boss know your planning to bring a woman you know just 3 months back to the USA with you? Tell him I am sure you won't mind my husband contacting them about this because we need to know more about your plans and how this works.
I would also tell them that you have the contact details of the USA boarder force and that they like to know that he is planning to bring your step daughter into the USA and she will overstay her 90 day tourist visa. Tell him his boss will be told this as well.
Let him see that your on to him. My feeling is that she won't be attractive to him once you do this. He is just looking for some quite woman who say yes to him, marry him so he can get married quarters and have his kids. Meanwhile your step daughter is left with no family or friend support and no money of her own living in the USA.
He is setting her up to be used and she can't see this. The fact he does not like her seeing her friends it a big red flag as her friends would be asking her questions and telling her not to move with him.