noticing some water leakage from my townhouse neighbors downspout, how to diagnose the leak and blame my neighbor

I noticed some water leak marks on my wall under my roof corner and went up and found this. My townhouse neighbors downspout leaned over onto my property line connected to his drain hole with a big tar ball. If you look at where the grate is, it almost looks like he cut a notch into my roof. There is no elbow or downspout connector, just a big ball of roofing tar.

I suspect that there is some leakage around here making it into the edge of my roof, or perhaps that the water is just soaking into my brick wall at that tar ball since if it's not fully sealed and soaking into my exterior roof wall.

Is it obvious from the picture what is wrong and how I should explain it to my neighbor to get him to fix it? Should I just have him move it back to his property line side and I fix the remaining void that is left over, or should I get him to fix it?

For reference, here's a correct downspout gutter that I've set up on my other side of the roof, notice the gap and space between my downspout and the neigbhor's wall along with a nice corner edge?