Well, now here is an update where the owner has cleaned off the corrosion,
showing some sound remnant metal, with the joint being apparently copper to
copper:

http://67.207.131.7/poolheater2.jpg

Here is an idea I had: could one not just wrap this snug with flat copper
braid (like copper grounding strap stuff) and then fill with braze, like a
solid metal "bandage"? Or even wrap with copper wire and fill?

Otherwise I am thinking a sleeve machined to fit closely over the remnant
geometry.

In any case, brazing will require opening this joint, cleaning off the oil
residue, reassembling with repair parts, filling with inert gas, and
finally heating to braze temp. Then evacuation and recharging.