Dahlias are perennials, they are just not very hardy perennials. It is possible that your mother in law has a dahlia that survives the winter. Is it planted against a south facing wall? Is it mulched heavily and kept dry over the winter? The main issues with overwintering dahlias are soil moisture level and frost penetration depth. If the ground is kept dry and frost free at the tuber depth then a dahlia should survive the winter regardless of your climate zone. If the soil is very dry it is more difficult for frost to penetrate than if their is moisture in the soil. I would expect that in much of Utah there would be very dry soil.in winter which would help insulate the tubers. A thick layer of snow that is on the ground continuously during the coldest months would act as an insulator too.

If you can have a picture of the plant, that will help confirm if it is a dahlia and possibly even its variety.

As for what works and what doesn't, others in my region also say that dahlias should be brought in for the winter, but I have had 2 years of dahlias returning from being in pots. This year, maybe the winter was too rough or perhaps last year's growing season was not good enough for them, but they did not return. However, the dahlias I had in the ground are back, so I suspect it was a container issue (I also lost 3 amaryllis bulbs in the similar way -- all the ones in pots are gone but the ones in ground are OK)