If you want to repair the crack just make sure you grind out all of the crack down to solid metal. From that angle it looks like you'd be removing the entire weld
The pic doesn't show the story. The crack is actually at the edge of the weld in the start of the HAZ. It goes perfectly perpendicular to the face of the frame and when it eventually cracks out if left alone, it will look like you took a laser and went around the edge of the weld to remove it.
All he has to do or would if he was going to leave it there is get the edge of the weld clean, and throw a nice bead in that has the crack as the middle of the fillet. It won't be any better or worse than the way the factory did it and the only way to stop another failure of the exact same type at the edge of the new weld is to brace the back of the mount over to the opposite frame rail.
I'm not disagreeing with your assessment, I'm just relating what I know after having repaired many of them and seeing the failures on the trails.