Your buddy has done some nice Tig welds! On the roll cages we fabricate at work for race cars we do what you did on the rear dropout gusset: the gap at the end in the corner can help reduce concentrating weld tension from shrinkage. If there is multiple planes of weld, like in corners, by leaving a little gap on the gusset corner (particularly if it's a wrap around gusset), when one weld cools, it's less likely to form a crack in the corner! Maybe good accidental engineering?!