I think the crypto wallet is lame, and am disappointed the Signal folks decided to integrate something like that, but it's entirely opt-in. If she doesn't want to worry about being taxed on crypto transactions, she can simply not use that part of the app. I actually forgot for a second it was there until you brought it up, and I'm a daily Signal user.
I think her concerns are overblown, but it shows how incompatible taxable transactions are with a privacy focused app. The two things should be kept well apart.
[Edit] Politically, it kind of defeats the purpose as well. You want to be able to argue that you have a right to privacy when it comes to personal communication. You don't want to be in a position of having to defend the privacy of trading securities.
Separately, you've either misunderstood her position, or it's poorly thought out, and/or ideologically based.
What path would tax authorities use to ask Signal users (and only Signal users) if they've used cryptocurrency?
Tax law. In the UK, every single payment in cryptocurrencies, however small, is a taxable disposal that you have to include in your tax return if your total proceeds or gains from all investments are above a certain threshold.
I'm not ideologically opposed to cryptocurrencies and neither is my wife. She's just allergic to anything that could potentially raise tax questions.
Edit: On second thought, I don't own a business, so I guess nobody is going to look into my tax fillings with the same suspicion since they do not expect me to be doing anything funny with my accounting.
Doing it correctly is non-trivial. You have to submit a so called computation for each individual disposal, which can easily run into several pages.
The algorithm for working out the cost of a disposal is actually a pretty interesting test case for learning a new programming language or paradigm. Try implementing UK share identification rules in SQL for instance :)