If you happen to get audited by the CAA, where is the paper trail that these are your hours to log if your name isn't on the tech log?
When the owner says you cannot sign the tech log, does he mean to authorise the flight like flying schools do? Signing as PIC and signing to authorise the flight can mean different things, and if the owner is happy for you to put your name as PIC and countersign it with your pal, then that's the only way I could see you logging these hours for licencing purposes.