My rear fender was drilled to match a karate monkey frame buy it's previous owner. I had to get creative with the mount to make the same hole work on my wolverine, which ultimately ended in failure in both the mount and the bolt/washer breaking a hole in the fender.

Is there a good way to patch this thing? It's about nickle sized.

If your goal is just to minimize the aesthetic damage then try metallic tape.

I've had mixed results with the aluminum tape I used from work.
On one aluminum fender it peeled off quickly so I switched over to a reflective tape and made some stripes that were aesthetically pleasing.

Currently I have some aluminum tape covering holes on a different aluminum fender and it seems to have held up pretty well for 6 months.
On the second fender I prepped the surface with light sandpaper and then cleaned it with alcohol before applying the tape.

Step 1: Completely disassemble your bike.

Step 2: using your fingernails, construct a new casting for the fender

Step 3: pour molten aluminum into the casting

Step 4: buff with emery cloth for a nice shine.