So I found solution:
int count = 0;
private void customersDataGrid_Error(object sender, ValidationErrorEventArgs e)
{
if (e.Action == ValidationErrorEventAction.Added)
{
save.IsEnabled = false;
count++;
}
if (e.Action == ValidationErrorEventAction.Removed)
{
count--;
if (count == 0) save.IsEnabled = true;
}
}