Ownership Rights: Can Borrowed Property Be Considered Abandoned

My question involves personal property located in the State of: California

I borrowed a (pricey) guitar about 8 to 9 years ago. The guy I borrowed it from moved to Hawaii and I didn't hear from him again for years. Figuring the guitar was abandoned, I gave it to my little sister. She now considers the guitar her most prized possession. Now suddenly the guy is in Oregon and wanting me to give the guitar back. My sister won't stop crying - this guitar changed her life. Do I have any legal recourse here? Or is returning the guitar my only option, short of making a purchase agreement (he's asking for more than I can reasonably pay for a 8 to 9 year guitar).

Thanks in advance for your insight.

Ownership Rights: Can Borrowed Property Be Considered Abandoned