Sure, it COULD get re-zoned to commercial, but it also might not, Change of use (especially from residential to commercial) is not assured, nor easy. I've done change of use from one commercial type to another for individual structures and it is time-consuming and expensive.

or does the purchase of land in the area for commercial purposes change the zoning of the property due to there being other businesses around it and thereby the zoning commission rezones the property?
No. the fact that you purchase a residential property, means you get the right to build residential structure(s) on it of a kind that meets zoning laws. You might WANT to build commercial, industrial or anything else on this lot, but what you want doesn't enter into it. You would want to be sure application for change of use was completed BEFORE you purchase it - as it might be refused and/or might never happen.

I'm told she's desperate to sell it for personal reasons.

You are being sold a bill of goods by someone who wants to unload this property.