Mother in a coma has been denied medicaid benefits because the nursing home said that her benefits have been exhausted.

No - there is no upper limited on Medicaid - if she is on Medicaid, her medical care should be covered.  Depending on what coverage she had - she could have exhausted her Medicare benefits, and now needs to apply for Medicaid, they are two different programs.  You need to ask for clarification from the nursing home.