The story that I've read was that IBM had the hardware but didn't think they could make software on time so they went shopping for the OS. First they went to Digital Research who didn't want to do it. Microsoft jumped in and promised to deliver what would become MS-DOS. The rest is history.
In no way was Gate's mother instrumental in the process.
Citation: http://forwardthinking.pcmag.com/software/286148-the-rise-of...
"In 1980, she discussed with then-IBM Corp. Chairman John Opel, who was also on the United Way committee, the business IBM was doing with Microsoft. Opel, some accounts say, knew little about the venture, but mentioned Mary Gates to IBM executives who introduced Microsoft at a meeting of IBM's top-level management committee a few weeks later."
http://articles.latimes.com/1994-06-11/news/mn-2837_1_mary-g...
The episode is a textbook example of "social connections" at work.
That does not mean she interfered. It's just a reminder that we should be careful about blindly accepting published accounts of events where the full details are only known by a small number of people all with their own agendas. This of course applies to everything.