Can't register car because dad cosigned?

My girlfriend is trying to register her car, but she can't, because her father who co-signed to get the car from the lot and is on the title, they both have to have be clear of any property tax they may owe, but apparently her father has a ton of debt in taxes. He refuses to talk about it and to help, and my girlfriend basically gets no help from the DMV. Is there anything that we can do to get her car registered without her dad changing his entire personality and paying off his debt? Thank you in advance.