I don't see how. The insurance company won't know anything about it, and in a dual-control plane the PIC can sit on either side, so the position of the two bodies in the wreckage won't reveal anything
Personally I wouldn't do it without disclosure to the insurer however, because if there was an incident and the "student" was injured, and his lawyer found out that going after the pilot for negligence will result in a bigger payout, then has an incentive to at best spill the beans and at worst tell a few lies. One cannot rely on "friends" in a situation like that. Might backfire though, because of the insurance was voided he won't get anything at all, short of driving the "instructor" into bankrupcy.