Pilot A: owns a plane and does aerial photography for a business, but
does not have a current medical or BFR.

Pilot B (me): current private pilot w/medical.

Can I legally fly Pilot A's plane on an aerial photography flight (with
him in the right seat) if I pay half the expenses, which in this case
would just be gas? Sounds like it would require a commercial
certificate to me but I thought I'd throw it out there.