I think we are really talking the same thing here, just in different words.
I guess I wasn't aware of the repair sections of the frames, or if I was, I
had forgotten.  But now that you mention that, I believe there were sections
for the C4s also, but the access to repair was terrible.

The way the hydroformed frame rail is made does not allow itself to be
restored by pulling and bending back to the original shape.  The initial
purpose of the hydroform was to eliminate the bending and hardened spots of
the old bent and welded process of making frames.  It is in many ways more
like a piece of tubing than a channel. If you have ever bent a piece of
conduit the wrong way so that it kinks, you know you can never get it strong
again and the kink won't come out in straightening.  In electrical wiring,
that is insignificant, but where people use conduit for awning frames,
tents, and such, this is an issue.

However, as you said that Chevrolet has made frame repair sections
available, Chevrolet has skirted the issue of traditional straightening for
repair and allow removing these sections that are forever weakened and
replacing with a new section.  This is why the insurance would give a good
rating on repairability.  And time is money, so the access and time to
repair a C4 frame would leave it with a terrible rating.

So both of our advice is right.  He should check the frame for kinks. This
would be evidence of a wreck and straightening, however, not the correct
way.  Any welded in sections would be evidence of a wreck, but if the weld
quality looks good and the car drives out right, then it should be good to
go with the only negative being you know it was wrecked once.