It's unknowable. But keep in mind that Microsoft was already established and very successful at the time, and Gates was a very astute and aggressive businessman. IBM went to Digital Research first, and Gary Kildall suggested they check with Microsoft. Microsoft was the biggest player in microcomputer software at the time. Why wouldn't they check?

And how much credence would IBM give a mom (who had little knowledge of computers and software) who suggested checking out her son?

Lastly, the accounts of her advising IBM to use her son's company are not verifiable.

IBM went to Gary Kildall first.