The issue is that the US can determine she didn’t file, file for her, and assess fines/tax. Once that gets to $50k she can’t renew her passport, and can no longer enter or leave the US as the law states she can’t use her foreign passport if a US citizen. If she lives in a country that assists the IRS with collection, they can reach in and take it. They have all her bank details from FBAR filing.

I know a US citizen who hasn’t filed in 34 years. I wonder what the penalty would look like if she ever takes a job in the US again.

Does she earn much? If her income is low enough, she may be exempt from filing.

https://www.hrblock.com/tax-center/income/other-income/how-m...

(Note that I am not a tax professional and you should not rely on anything I say.)

It doesn't matter. If she has bank accounts abroad exceeding the threshold, she's obliged to file just to disclose her foreign bank accounts.