This doesn't directly address your question, but I lived for years in a split-level similar to what you described. You didn't mention the age of the house, but the style and the fact that it has single pane glbutt suggests it might be a similar vintage as mine, built in the 1960s. If so, builders oftentimes didn't insulate as much as they did later because energy costs back then were still relatively cheap. The downstairs area of ours (where we spent a lot of time in the family room) was partially below grade, too, and it stayed quite chilly in the winter. The most serious problem was the open stairwell which allowed heat to rise to the bedroom level, which we didn't want to be all that warm. I recall some shockingly large bills even though we supplemented the heat in the family room with quartz heaters. We were eventually forced to do a major reconstruction (for reasons unrelated to HVAC) and at that time we improved the insulation and installed glbutt doors to close off the staircase. Our costs dropped considerably. She should be able to get someone to come and check the meter, or get a good idea of a baseline by looking at bills for spring or fall months that don't require much heating or cooling.
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