I'm a creative who's has made multiple ips, but they have all been in the conceptual phase. Recently my sister had a messy break up with her bf, after about a year knowing eachother. I had created an ip before they had met that I believe is one of my most potentially successful ideas. While in their relationship, he created a note book on one of his own ips. Me and my sister work together and are very proud of our works, so we often talk about them with friends and family (I know, dumb) so he was aware of the property I made.

His Ip has similarities to mine, though not enough to really consider as stolen.

After they broke up he left behind a lot of his stuff. Including that note book. While I'm confident that my property and his property are distinct enough that it isn't infringement, he is particularly petty, and a prolific liar. I'm worried that since we have his iP, he could in theory come back and try to copyright our ip.

How can I take steps to safeguard our ip incase he were to try this? What should we do with his notebook? What legally can I do? Am I able to throw it away? I have no interest in reading it nor taking inspiration from it. I just want it and this potential problem out of my life. But I know that it would be dumb of me to assume that a man like him wouldn't be capable of making the production of my ip an nightmare.