How often is she feeding?  Does she stimulate her breasts after feeds with a pump.  Has she ever pumped her milk to see what kind of quantity she gets at a time.

If she has been using a shield all this time, with no supplementary stimulation of her breasts, either by hand or with a pump, it is definitely time to start additional stimulation with each feeding.  I would also feed infant with a cup, or if necessary, eventually, SNS, with mom's own pumped milk p each feeding.  If mom has no milk, it is definitely time for a regimen of galactagogues.  And in the mean time, FEED THE BABY.
Even if it means feeding some of that awful stuff that comes in a can or powder.  Awful stuff is better than no stuff at all.

Good luck,  Chanita, San Francisco