Thank you for all of the replies. I should have probably added more information in the first post.
The aircraft is insured for anyone with over 100hours, so insurance is not a problem. It seems that the owner is happy for my friend to book and pay for the hours as normal and then me go up flying (as PIC) with him and only me logging the hours.
The tech log is still being signed and the maintenance hours are still being recorded. I have no doubts about the integrity of the owner or the maintenance of the aircraft.
To answer the question about would I be permitted to fly the aircraft alone, I have the appropriate rating and am covered by the insurance.