To lower the frequency of her violin, a musician would need to lower the pitch of the strings, which can be done by tuning the strings down to a lower pitch. This isn't directly addressed in your options, but among the ones you provided, the closest reasonable answer would be to:
Adjust the chinrest and draw the bow across all strings equally.
This is not a typical method for lowering frequency, as the chinrest adjustment won't change the pitch. However, if she were to lower the pitch of the strings by tuning them down (or using alternate tunings), then drawing the bow across all strings would help facilitate playing those lower frequencies.
The other options (drawing the bow across only the thicker strings, selecting a shorter bow, or selecting a specific brand for low frequencies) do not directly lead to lowering the pitch of the violin.