The issue of pro-rata is applicable to private pilots, and very irrelevant to this discussion. If that's the tack you'd prefer, one may as well go even farther afield and tackle the issue of holding out, which would be along the same line, but equally as irrelevant.
The original poster did not provide enough information, save that the owner allows him along for the ride, just likes someone to "go along," and "takes him up" frequently. There is no indication that the flights are instructional. The only indication of instruction that might take place is theoretical and offered as a wave, a justification to find a way to legally log the time. Thus far, such justification has not been provided.