You can ask him yourself, if you want: rms@fsf.org
He replies to almost all emails within a couple of days.
EDIT: He actually answered a very similar question in his reddit AMA [1]:
22. two_front_teeth: Suppose your doctor told you that you needed a medical procedure to survive but that the procedure would require inserting a device inside of your body which ran proprietary software. Would you be willing to have the procedure done to save your life?
RMS: The only way I could justify this is if I began developing a free replacement for that very program. It is ok to use a nonfree program for the purpose of developing its free replacement.