I have been working as a rental sales agent (RSA) for some time (not with Hertz though). The motorbike license does not help at all in order to fulfill the one-year license requirement. After all you are renting a car and not a motorbike.
There are two points to this problem
a) having someone accepted as an additional driver
b) insurance coverage in case of an accident
For a) it is mostly up to the RSA and the rental system whether your partner is accepted as a second driver. The RSA will have to enter the license details into their rental system. The rental system then may or may not check whether the license to drive a car has been issued more than twelve months ago.
Especially with a foreign license and having a motorbike license for a long time they may enter the wrong date and the rental system will be fine with that. Also the RSA may or may not either look at the wrong date or not care at all.
Regarding b) if your partner is accepted as a driver and entered into the rental agreement and he/she causes an accident the rental company may demand compensation even if you have proper insurance. Simply because insurance only covers drivers that have had their license for more than 12 months.
So even if your partner is added to the rental agreement as an additional driver you are at risk if damages occur.
There may be hire-firms without the one-year condition but in my experience you will not find an international hire-firm without that condition. This might be different for local hire-firms but I wouldn't bet on that.