When you mother purchased the property - she might purchased the title insurance - please verify...
The title company should be responsible for any additional tax obligations (including penalties and interest) if in-fact the title was not clear.
If you mother did not purchase the title insurance - means - she took a risk for the situation that unfortunately happened - and in general - as the owner of the property - she is responsible for any tax liability related to that property.
If the title search provided incorrect information - I suggest to verify terms and condition for their service - and if they may be held responsible for damages.
Also it is possible that the county provided incorrect information to the bank and may also be held responsible till some extend.
But the situation is very different if she had a title insurance - and I afraid that she would need an attorney.