A friend of mine has an old steel tandem and the top of the seat tube broke. He's convinced that simply welding it back together would work. I haven't really worked on frames, but the break doesn't seem very clean and looks like it would be very hard to fix. Are thoughts/suggestions? Thanks.

Where is it broken? If it is above the lug or seat tube clamp it may be amenable to welding.

A little tough to see whats what, but welding what's there is going to work even if you did join it or cut it clean and butt something you will have to ream the seat pin again and it will probably last a short time. It looks like the seat tube passes throught the top tube. You could slip another tube internally butting it and use a smaller diameter post but the best thing is to replace the entire seat tube going oversize to make up for material lost getting the old tube out. Good luck.

I'm not sure what I'm looking at either, but it appears to me (FWIW) that operator error is at fault here. The only way I can see this kind of failure is if there was only an inch or so of seat post inserted into the tube.

Regardless, if I was tasked with fixing the frame I'd clean up the top of the damaged seat tube and braze in another section of tube on top of it, with a reinforcing ring around the break. Then I'd be damned sure to insert the seatpost several inches into the tube from then on.

The simplest fix, FWIW, is just to get a seatpost with an internal expander. Then you just have to clean up the ragged edges of the break and call it a day.