It looks like you've already got what you need to convince her... you're losing money by keeping it in cash. Now, you need to convince her that the alternative will not do so. Maybe explaining the perpetual money printing by The Bernank would be a good start on that path.
People need to start using their brains. As long as Americans keep doing what is "traditional" instead of what makes sense, we're going to continue on the road to serfdom... just educate her on the issue.