I think one issue here might be that principles/moral convictions are not yet equated with religious or gender-identity rights. Which kind of makes sense, because otherwise people could take advantage. But it leaves us in an awkward position where a certain moral stance that is extremely strongly held, perhaps even to the point of being nearly as much a part of the fabric of your being as your identified gender or religion, has little or no protection whatsoever.
I suspect a lot of this issue depends on your job description. As a social media marketing manager, it would potentially not be as easy to refuse to use smart devices or social media - although anyone should be able to refuse to appear as a face in a promotional video, especially within Europe, as this is a matter of consent and personal rights.
I am fortunate enough to work for a small-enough firm that I can tell them my limits and boundaries. They know that I will not receive, let alone answer, any attempts to reach me via Facebook Messenger or WhatsApp, even though I am often grumbled-at for it. I have also made it clear that if an app or smartphone is ever required for me to do my job, they must provide it and expect it not to be operational outside working hours. I do not install work related things on my own devices, partially because they are my personal devices and I pay for and maintain them; and partially because I don’t want to have the responsibility/liability for my employer’s information in the event my device is compromised. If they provide the device, and I just use it however they instruct me, they are responsible for its security and any hacks/breaches, and I am not liable.
I think your friend’s stance is entirely reasonable, and I wouldn’t be surprised if her employer’s stance was illegal, even in the US. After all, someone could easily be part of a witness protection programme, whereby a TikTok video could easily compromise their very life. Additionally, if activities such as “featuring in promotional materials” were not in her original employment contract, I fail to see how they could enforce this request. What next? Suddenly mandating that all female employees attend work in the new uniform of a skimpy bikini or wet T-shirt, as “team-building”, so that male staff can “get to know” them, and also so that “promotional videos” can show how relaxed and informal the firm is? Where would it end? Demanding staff prance about in embarrassing videos is not a reasonable or acceptable request for a normal employer.
If my employer did not respect my personal privacy stance regarding social media, there’s absolutely no question: I certainly would quit. But I am in the fortunate position where I have enough savings and qualifications that I could take the gamble that I would find another job before losing the roof over my head. Someone struggling and living “hand to mouth” might not have the privilege of that choice.
We can protect ourselves somewhat by making it clear from the outset that we do not wish to appear on promotional photos, videos, social media and so on. If an employer knowingly takes you on when you have made this clear, you always have that argument to present when faced with an unreasonable request. I do not know how the law systems in different jurisdictions would interpret such things, but I’m sure having an e-mail trail where an employer sees (or even acknowledges) that you don’t want to appear on TikTok, is surely a good thing.