I went with a friend to see a standard 1969 Camaro that he is considering purchasing. I noticed that sections of each side of the subframe had been cut off and new sections welded on. I'm assumed that the top of the frame where the bolts attached to the body had rusted through. The owner indicated that was the case and that the repair had been made in that maner over 15 years ago.

The welds looked good, but I'm just wondering if this is an acceptable way to fix that problem? Told my buddy I'd ask the experts.

I'd pick up another subframe. There are plenty of decent ones out there, so your friend should not need to settle for one that has had the frame horns replaced IMO.

Questionable...
how was it cut?
who welded?
how good are the welds underneath?
is it straight?
Too many variables and for future maintaince this could influence things like column steer box alignment, wheel alignment, suspenction geometry.
Cost up a replacement subframe, r and r parts etc
Add quite a bit then make a reduculus offer and be prepared to walk away.
Unless the price is rediculusly low, and your friend knows his way around with a spanner..wouldnt touch it.

If it have been done well you wouldnt have picked it up unless looked inside the subframe..that makes it even more suspect.