Never seen one mounted vertically like that, no wonder it cracked!
Looks simple enough to repair...make a temporary wooden spreader to keep the rig in place, then take the aluminum one to a competent welder to run a bead along the crack. Should do the trick. If you don't feel that this is strong enough (should be), wrap a band of aluminum, say 1" wide by 1/8" thick, around it and weld the band to the spreader at the very end near the damage. This will thicken the end substantially.
Now the question is, why did it crack in the first place? Has the rig ever been pulled? When handeling the spar, they could have bumped the spreader into something.....