Of course one often hears horror stories about unreasonable officers, but the vast majority are reasonable and will recognize that your mother flying to Toronto is reasonable, whether because of cost or because of other considerations. In the unlikely event that they challenge you, stick to your story -- the truth -- that it's more economical, that it's more convenient for her not to have to take two flights, and whatever other considerations factored into the decision.
For many places around Buffalo and Niagara Falls, it's probably also faster because having to change planes to get to Buffalo adds more time to the trip than one would need to drive from the Toronto airport.
I have some friends in Kingston (Ontario), and I've visited them on couple of occasions from New York City by flying to Syracuse and renting a car, because flights to Canada were something like 3 or 4 times the cost of a flight to Syracuse. One of those times, the Canadian border officer raised his eyebrow and said "Arkansas?" I had just enough presence of mind to remember that the car had Arkansas license plates, so I said "it's a rental," and then explained that the $100 flight plus the car rental and 2-hour drive seemed like the better option, and that was it. Since you live nearby, you will be far less likely to raise red flags. If you were bringing your mother in over the land border while being a resident of Colorado, you might have to have a more substantial explanation.
On one of my visits to Kingston, when we returned to the US, the officer looked at my passport and asked me what I'd been doing in Turkey -- where I had last been over a year previously. I think he asked because he couldn't ask my wife (she works for the UN and has "semi diplomatic" immigration status), but since I'm a US citizen I'm fair game. I would have pushed back -- pleaded the fifth or whatever -- but we had a plane to catch so I answered "vacation." That was the end of it.
I suppose what I'm getting at is that the officer may throw some things like this at you to see how you react: questions that could be seen as implying that something is afoot even though there's no reason for suspicion and the questions don't necessarily imply anything. They will be looking for a defensive reaction or signs of stress or nervousness. If you're nervous about your mother entering by the land border, and if you show this nervousness in response, they may pick up on that, and that is more likely to raise a red flag than anything else.
If that happens, just be honest about the reason for your nervousness and secure in the knowledge that there's absolutely nothing wrong with your mother's travel plans. Better still, be secure in that knowledge and realize that there's nothing to worry about.