I am an Apple geek since 1980. The problem isn't tech phobia/lack of tech skills. I have had the problem described with Touch ID on every single device I have used Touch ID on--regardless of the shape of the sensor (including the power button sensor on the iPad mini). It will work fine for a few days and then it will stop working.
In my case, I've always attributed the problem to the fact that 50 years ago when I was in medical school, we did not glove to do gross anatomy or neuroanatomy, meaning that we spent all of our first year bathing our hands in formaldehyde for hours a day. As a consequence, I think I have very shallow fingerprints, prints that have essentially gotten "burned off" by all that formalin exposure.
FWIW, I've also had problems when in-processing for federal jobs that require fingerprinting--they have the devil of a time getting good prints. This was true back in the old days when prints were taken using paper and ink and even more so with electronic print readers.
Perhaps OP's friend is experiencing something similar due to solvents used in making jewelry.
In any event, I find it a tad encouraging to find that other people are experiencing the same thing--I've always felt like I was the odd man out. I certainly hope someone here has tripped over a durable solution. Having to add a new fingerprint every week or so gets old. In fact, I have stopped using Touch ID; I don't even bother to set it up on my Macs, preferring to log in with a password or my Apple Watch. I much prefer Face ID over Touch ID.
