Co-Signer Rights/Is Legal Action Possible?

Hi, I'm seeking advice to help my dad. A few months ago, my dad (unbeknownst to me), co-signed an auto loan for my step-sister. Now, my dad is getting calls from the lender saying that she hasn't made her payments. My dad, knowing this will affect his credit, paid the monthly payment and has since tried contacting my step-sister. He doesn't know where she lives and she hasn't answered any calls, messages, etc. for weeks now. Not from him or her mom (my step-mom). He knows where she works, but he doesn't want to have to approach her at her place of work to see what's going on. He knows that she is okay, as she is posting on Facebook and texting other family members (one message being about how my dad is a p.o.s), but won't respond directly to him or my step-mom. Can my dad take any legal action to ensure she pays her loan or remove himself from it?

Background Info: They've never had a great relationship (step-sister and dad). My dad and step-mom got together when all of us kids were 18+ and in recent years my step-mom has had some medical issues that have prevented her from working. My dad is the only one who works (60-70 hours a week) and takes care of her full time. Both of her children who live in driving distance only see her a few times a year. My dad has helped my step-sister numerous times in an effort to make my step-mom happy but it just keeps screwing him over. As his only child, I'm getting really frustrated to see this happen.