A well-designed high-performance structure has one defining characteristic, that the entire structure is highly-stressed.

Most frames have higher and lower levels of material stress throughout the structure, so the highest-stresses areas will fail first while the balance of the structure will have accumulated fatigue degradation before failure.

It's known that aluminum has a more sharply-defined service life in terms of fatigue failure, as happened here.

This frame is literally worn-out in terms of fatigue-limit service life, and repairing it to go additional years is somewhat dangerous.
Any testing that was likely done before manufacture would have been given the quicker go-ahead if/when the first failure was relatively fail-safe as was the case here. Repairing it and continuing on with service would make failure of some less-failsafe part more likely, such as a head tube joint.
Welded aluminum frames have been heat-treated after welding, and re-welding without again heat-treating is just asking for more-rapid failure, not to mention the difficulty in maintaining alignment after welding both chainstays and in maintaining a round-enough bb shell after any welding was done at that junction.
This rider is large, and would be best advised to look for a newer frame imo, and it appears from the OP's post that such decision has already been made.