How to flash a gable roof butting against a brick wall?

Hey all. My mom is dealing with a leak issue at her home. A couple of decades back, they built an extension on the side of their home. A gable was added (I don't remember it existing prior) to direct water away from the two-story brick wall where this portion of the existing roof was. [Here's a terrible drawing](https://imgur.com/a/YuY4kgq) to hopefully clarify and illustrate the situation. The red line indicates the sloped end of the shingled gable where it attaches to the brick wall. Somewhere along this left line there's an exposed area that is allowing water to enter the house and leak down the interior wall.

I haven't gotten on the roof yet to inspect things, but I do remember previously seeing something like an L-channel where the shingles and brick meet. Can anyone point me to perhaps a detail from GAF or Carlisle or some other source that will illustrate how this gable termination is supposed to be properly flashed? I'm speculating I would need at least two different flashings: one L-channel that butts against the brick and slides under the shingles along the brick wall and then a Z-channel or drip edge that goes on top of the L-channel? Or maybe the L-channel alone is all that's required so long as it's properly caulked on the side that butts against the brick?

Any help would be greatly appreciated. Thanks!