O-kay... I've worked in highly sensitive areas of government, and have friends I know to be in other highly sensitive areas.
Addressing each point in turn:
- Uncontactable in a meeting/day or so is possible, if your friend means she can't have a nice chat, but though your phone etc. can be locked away, there will always be a way to get an urgent message through from a partner.
- It IS possible to be away/abroad for weeks at a time, but again there is always a way to get a message, even via a line manager, through the local embassy, etc. Sometimes we travel with burner phones/computers, but that's no excuse for going entirely off the grid. We still have communication channels. In fact my DH is part of my safety mechanism. If I don't contact him as expected then he needs to alert my line manager, it's part of the risk management. My DH isn't British either btw, in fact there are lots of international marriages in my team. Sometimes we can't tell wider family or friends where we are going, BUT DH always knows which country I am in, how to get in touch with me for check ins, and how if necessary he can get a message to me/raise the alarm in an emergency.
- Apparently he can’t tell work about their relationship because she’s originally from America (lived here since she was ten, though doesn’t have a British passport), so their relationship is secret from his work and even his parents (she’s never met them).
Bollocks, I'm afraid. If this bloke is on any kind of secret job, or even a middling sensitive government job, he would not have got the job without something called vetting first. There are various levels of vetting, depending on the sensitivity of material you will be privy to. If his job is secret/sensitive to the extent he is suggesting, then his entire background and life is investigated. It can take months, they'll be going through his bank statements, line by line, they'll have interviewed his partner, parents, mates, old school friends etc. Even if he, for some reason, wanted to keep her secret because he was worried it would prevent his getting accepted, they would have found the evidence of her during the checks, and would have followed up. Regarding foreigners, most nationalities are not a problem, (though they get vetted too, of course). The US would not be seen as high risk.
- he is invisible on social media, has no account on anything.
Even people in the security services have social media accounts. They just keep them locked down and are very careful about what's on there.
- literally home 1 weeknight every three weeks or so, sometimes more.
Again, she should know vaguely where he is and have a way to contact him. And I don't know of any jobs with this kind of pattern - you're either working normal hours, or shifts, or posted abroad on a permanent basis, or making periodic visits abroad, but this pattern just seems highly unlikely.
- she’s never met friends or family
Major red flag, there's no security reason why his partners wouldn't meet friends or family.
- he claims to be a spy or whatever. (Not technically a spy but works for the government and can’t claim their relationship??)
BOLLOCKS (see above)
If he's in middling sensitive work then he'll probably be able to name the actual department or agency he works for. If it's highly sensitive he'll say, if needed, that he works for a real government department or agency that provides cover, such as FCO, DfID, DIT. Not giving away any secrets here, btw in case anyone's concerned, this is standard knowledge.
- says he gets no holiday and also works 7 days a week, every week.
We all get holiday, in fact holidays allowances are quite generous :-)
- claims to work 24 hours straight sometimes and that’s why he sometimes goes off the radar.
Hmm, sometimes we can work LONG hours, maybe for a couple of weeks with little downtime, or a whole day and much of the night, but as mentioned above, we are always contactable in an emergency.
DH, a foreigner, knows what I do, though not the details. We're not allowed to talk specifics, but I can let off steam, he can support me etc.
Didn't see the question about accommodation referred to above, but certainly the government pays for my travel and subsistence expenses when I'm away with work ( a government salary wouldn't stretch far if we had to cover our own flights, that's ridiculous!!). And booking is mostly done by the travel team, if I (occasionally) have to get something myself, then it's refunded in a couple of weeks. If this is his excuse for not sharing money with his partner and child, it's utter bollocks.
Sorry, none of this adds up at all.