Either there is some residual bug still in there, or her flora and fauna (good bugs) have been depleted.
You may try activated charcoal to sweep out the bad remnants and probioitics to repopulate the gut. Give the charcoal after each episode of vomiting or diarrhea, and give the probiotics at least 2 hours before or after the charcoal.
Some moms have had great success giving Tummy Tuneup immediately after vomiting and diarrhea, so you could skip the charcoal and just try the probiotics.