You're right, it should have an "L" shaped head flashing at the top. It should extend beyond the C (we call it J metal but no matter) a little. It should be bent a little less than 90° so the water heads to the front of it. Also, we would bend up tabs (end dams) on the ends of it so that the water has to run off the front and not dump into the J metal on the sides. That J metal needs to be lapped correctly at the bottom too i.e. so it will shed water and not buck water. I can't see the bottom so I don't know what he did there. I hope I haven't confused you too much

,
These are the only pics I have that kinda show it. The first one you can see the head flashing pretty good. It's an overflow drain not a window but the same principal. The other pic is kinda far away but we did the same thing around those two HVAC units that stick through the wall. I took these in the equipment wells on the roof of a 28 story building in Bellevue, WA.
As for whether to call him back or not I would say if it leaks he should fix it. If we did it we would have to fix it. More sealant is not a good solution.
I can draw an exploded view if you want.