GTAW that case if he's your buddy, works everytime and have repaired many of my own bike parts in the past with this process. Make sure you clean the case it real well, oil will cause pin hole porosoity and make welding miserable, Chemical or steam cleaning usually does the trick.

One more thing, wether use braze or weld, get in and get out or the case will warp big time and create more problems than when you started.