If the serial number was just in a plaintext file it’d be too easy to hack. Adobe makes their licensing very complicated in an attempt to thwart pirates. It may be in the registry or in an encrypted file. If you still had your laptop and it was a Windows computer, then there is a program you could have run to find the serial number, but without still having the computer with the software installed on it I don’t think you can find it, yourself. The easiest thing would be contact your friend, but maybe that is no longer possible. Your friend owns the product and you didn’t otherwise document what it was. If you have a disk image of the laptop then maybe, but otherwise, no. I think you’re out of luck finding the serial number in a simple files backup.
