If you want a technical solution, you have to demand that a person always contact you from their own device as "2FA", or at least that they some kind of 2FA device on them... except there are a billion ways that somebody might lose access to such a device when they were in genuine trouble, and scammers are totally capable of making it look like one of those scenarios has happened.
You're really down to the point where people have to do "mind to mind" authentication with shared secret knowledge... while under extreme stress... in a case that's uncommon enough that most people will not have practiced it.
This is not so great.
Not sure enough to feel really comfortable betting her life on it, anyway. Not if we hadn't drilled it on a daily basis for weeks and a weekly basis for months.
It's easy to blank on things when you're adrenalized, say if you've been kidnapped. And it's also easy to blank on things when you're adrenalized because you are hearing the person's voice saying they've been kidnapped.
... and if I asked a scammer pretending to have kidnapped her to let me call her on her phone, I would expect to get the obvious reply: "I threw her phone away. I'm not dumb enough to let you track me/her/us through it". Which is totally credible because that's what a kidnapper should do.
When you get the call, the strong prior probability is that the whole thing is a scam, but that's not so easy to hold onto in a situation like that. And even if you do hold onto it, you will be scared.
Oh, and on edit: Yes, I expect I would keep it together enough to call her on her phone to check, since if she hasn't been kidnapped there's nothing stopping her from answering it. I don't know if I'd expect that of others. But it's also true that if I call her, and nothing is actually wrong, I still expect about a 50-50 response rate because she doesn't hear the thing, has it on mute, or is in school and forced to keep it in her locker, or has let the battery run down, or whatever.
I don't think it will be long before video and audio is no more convincing than text is now. We will stop falling for the AI scams, just like we (most of us) stopped falling for the Nigerian princes, scam ads/virus popups on the web, and those fake emails from family members claiming they need us to wire money so they can pay for a flight home or whatever. Basically, my thesis is that people will start to get wise to any scam that is sufficiently common and harmful.
Real kidnappers might have to learn to work to convince the families that the kidnapping is real and not just another scam.
Every piece of communication will have to be signed to verify authenticity.
Camera makers are already looking into embedding authentication technology at the hardware level.
It's an inevitability.
If it's someone you know, with a reputation that makes them believable, then that's basically a criminal act and depends heavily on that person's reputation. There should be enough deference even without that, beyond simple harassment (which is what you're saying, where you lose your job or relationships).
Otherwise I'm not sure why your spouse or boss would rather believe some video coming from [email protected] is more believable than the person themselves.
Anyway, I was talking about overt scams not the wider cultural/social implications. This is a large new burden that will be imposed on society. We will learn to adapt and society will manage.
It's impractical to have authenticity enforced by millions of consumer devices. If any random manufacturer in a third-world country can generate millions of valid signing keys to be embedded in millions of cheap phones or cameras, then there are many employees there that can also leak a bunch of valid keys if the price is right, and these keys can sign anything in a way that's indistinguishable from all these cheap cameras. And if you revoke the signatures of any "untrustworthy" manufacturers (really, any manufacturer can be and will be compromised, especially if some governments want to manufacture propaganda), then people won't stop using the phones/cameras just because their signatures aren't considered kosher, you'll still have millions of people uploading genuine, valid images from these cameras, so either people will have to trust invalid signatures or refuse lots of benign content, and I'll bet most people will choose the former one.
And of course a camera doesn't know if it's taking a picture of reality or another picture - it would take a relatively simple optical setup to allow any camera to record (and sign) an image off of another screen, so if some authenticity-verification system was actually working and popular, any reasonable criminals would do it and have signed recordings of their deepfakes from a real major brand camera.
Most young people these days aren't even using phone numbers/SMS these days except when forced to, so the threat will remain with the older generation and either die off with them or until the phone companies solve this problem... Or some digital voice system replaces it.
Otherwise it's just email phishing. Anything involving money transfers is already locked down and mostly a matter of fooling people... Which again is mostly older people with a poor grasp of technology and common threats (modern street smarts).