**Her initial thoughts were to just pay off the mortgage and keep the rest in savings. Of course, I told her that savings will not accomplish much for her, so now I've turned her onto investing.**

Not enough info to really say, quantitatively. If it is a fixed-rate, amortized mortgage, it really depends on where she is in the amortization schedule. An amortized mortgage has the bulk of the compounding effects front-loaded. For instance, if she just created that mortgage last month, she'd save a whole lot more than she would save if if she was in the last 5 years of a 30-year mortgage.

Then you'd have to contrast whatever effective savings that comes to with the projected compound annualized return of investing in equities, which would be significantly more than a 4.5% APR. It's the gap between the two, and the investment horizon, that should be informative.

But regardless of where she is in the amortization schedule, equities present a better risk-adjusted return. But that doesn't matter one bit if she has no tolerance for risk. That is the prevailing criterion.

No tolerance for risk = pay off the mortgage completely.

Scale any variations of that tolerance to a % of payoff (or no payoff at all) as appropriate.