My typical approach is to explain that there is no such thing as “risk-free.” If you invest in the market, you risk losing money in a dip. OTOH, if you keep your money in “safe” investments like cash or CDs, you risk losing buying power to inflation. And unfortunately, the latter risk is almost guaranteed. So keeping your money in cash is trading a risk of loss for a guaranteed loss - all for an illusion of safety.
But it sounds like you have already had that conversation, many times. So if she isn’t willing to listen, why continue to talk? Why not just say something like “sorry, doesn’t exist,” or “sorry, can’t be done” - and then change the subject? You can’t help someone who refuses to be helped. She is a grown-ass woman, and she is entitled to make dumb-ass decisions if she wants to. Especially since there doesn’t seem to be a real risk to her, given the amount of money she has saved. Just let it be, tell her she’ll be fine when she frets, and don’t engage.