If your welding friend can weld the cracked top tube area 1st, plus weld a wrap around gusset around the seat tube and back over both sides of the re-welded top tube it might hold up. The trouble is the frame will have originally been heat treated in production, and re-heating during your welding repair may actually weaken it second time around.
I have seen this extra gusset approach done a few years back on my team mate’s downhill bike in the same place and it held up for another 18 months abuse before failing again at the new weld.
The alternative is to search ebay for a bargain mainframe to replace it?