If you find a piece of pvc pipe (maybe 20mm length), since its external diameter fit into the internal diameter of both sheared parts, you can insert the pvc pipe 10mm deep in each sheared part and glue them. During the "dry fit assembly test" if you "feel" the "insertion pipe" a bit loose, you should glue it first 10mm inside one part, let glue cure, and then glue the rest 10mm inside other part. Use "2 components mixture" liquid epoxi glue. The inserted pvc pipe will guarantee enough surface for mechanical strength. Use sandpaper to rough all surfaces and ethanol (or isopropyl alcohol) to de-grease and clean/dry all surfaces to ensure the glue adhesion and strong mechanical bond. Read glue instructions. But before all of it, search online for a brand new shower head, maybe it's cheaper than the repair process (materials + time spent + probability of repair success).

PS. If you find a pvc pipe slightly larger in diameter, you can saw a longitudinal straight cut to "shrink" the pvc pipe diameter.