In the nicest possible way, no technical measure can account for a human making a very bad decision.
Which is why draining a retirement account shouldn't be just behind a technical measure but require a lot of very explicit authorisation. Not wanting her to have the financial apps is basically identical to just not wanting her to have a card or access to the account because you don't trust her. Whether that's justified or not, it's not something that anyone else can do anything about, and it's not related to the security of the app itself.
I had a friend in my former workplace, he was old and "didn't want to learn" all the new technology by his own admission. He had a very old PC, he deliberately kept a non-smartphone, he owned almost nothing beyond a TV really. He would hang out with us, the IT team, a lot for other reasons but he never really got into the IT and didn't like gadgets, so we ended up doing a lot for him and talking him through many things. I'd even do things like buy stuff on Amazon that he needed and couldn't get cheaper, then he would give me cash for the purchase. And things like top up his phone online for one when he brought me a receipt from a shop where he'd paid in cash to get a top-up code.
And one day, after a particularly long rant from him, I took him to one side and said "You know? It's never going to get LESS technological? Nobody's ever going to go BACK to pen and paper, or cash machines, or little rubber stamps, or whatever else. It's only ever going to involve more technology and more devices and more new routines." He brushed it off at first but then a few days later he came back. "I've been thinking about what you said ever since," he said, "and you're right".
With our help he bought a new smartphone (nothing fancy), a tablet, a PC and... it escalated... before long he had CCTV all over his house, was buying stuff off Amazon on his own (finally using his credit card!), and he got on-board. He's in his 70's. He had no particular aptitude for it beforehand. And it wasn't actually all that difficult. All he needed was the INCENTIVE to do so. The actual practicalities weren't a problem.
The problem you have is not insecure phone apps. It's an insecure person, who doesn't want to learn why it's important - important to be able to spot fraud, to check on her accounts regularly, to know how to deal with a scammer, to be able to remember passwords... these are all LIFE SKILLS, nothing to do with technology at all. They've been required since the days of remembering a telephone number or people knocking on your door claiming to be someone they aren't. If your wife is insulated from them, she never had a need to learn them... someone else "does that for her", she can "just check with" someone else.
My mother was the same for a while. Even managed to sign us up to an alternate electricity provider on the doorstep without realising (but it was soon discovered and rectified as we have consumer law in my country so "doorstep" contracts can be cancelled up to 60 days later and require a letter confirming the change, etc.). Because dad just did everything for her. She wouldn't use a cashcard, dad would give her cash. She wouldn't go online, because dad did that. And so on.
It's vital that you break that dependence... not just for technological or financial reasons. And the way to do that is that she must have the incentive to do so. She needs to be responsible for those accounts, and for other things now, not just able to rely on your, family, friends, etc. to do them for her. I always cringe when a family member asks me to help them log into the most personal of data they hold... no, I shouldn't need to. I'm going to teach you HOW to get in, and you're going to do it, repeatedly and over and over and you're to going to text me tomorrow and tell me that you got in on your own when you started from scratch in the morning without me there at all.
It's the only way to do it. Get her into some adult education classes. Because it's not the technology she's afraid of. It's the responsibility. And it carries over into so much of life that has no technical basis (e.g. choosing to walk down dark alleyways, etc.) that it's a critical life skill.
If your kid was that bad at technology you wouldn't just ban them from ever having their own bank account even in adulthood. You'd let them make mistakes and keep getting them to do it until they took responsibility.
I appreciate your plight, I really do, but it's time to hand over the responsibility (and with that the potential for making mistakes). And that means you stop worrying about it (in the nicest possible way, it's not going to affect you, and whatever was going to affect her was going to do so anyway), and at the same time you make it clear THEY have to pay on their card in the restaurant, no, THEY have to transfer the money from savings. No, it was YOUR responsibility to pay the gas bill, etc. You need a new phone, okay, you need to find one and a contract to go with it, and how you're going to pay that and where have you budgeted for that?
It doesn't have to be tough love, but you need to handover... same as you would leaving a job.