Ex Step Dad Is Trying To Sue Me And My Mom For A Student Loan He Co Signed On

For context, this man and my mom divorced when I was younger but my mother had decided to rekindle with him when I was in high school. By the time I headed to college we were all living together again. My mom’s credit isn’t good after signing lots of school loans for my two older brothers who didn’t even end up finishing college. So by the time I needed a parent to sign on my student loans, her credit wasn’t going to cut it. My ex step dad and her signed on a parent plus loan. My mom being the primary and my step dad being the co signer. This was November 2020 during my freshman year. Fast forward and a few years of his verbal abuse and emotional manipulation, my mom finally put her foot down again and left him for the second time. He got really mad and in true him fashion tried to retaliate against her for daring to leave him again. He first tried getting her fired from her job by calling corporate and making false claims. He then stalked her and constantly blew up her phone. He moved to Florida (which we all live in Nebraska) and we thought that would be the end. Until we got a letter saying he’s claiming to have been fraudulently added as a consigner to this parent plus loan. We are having a hard time with attorneys helping us and still don’t understand how he is making a case of this. Any advice on what we should be looking for to prove his compliance or what laws relate to this? He is suing both me (the student) and my mom (the primary)