I'm worrying a lot about this team member at this point, and what work can be trusted with them. None of the failed threads aside from the broken tap were noticed at all by them, which I immediately saw when I picked up the part. They also said something weird to me while they were doing to work; that they were having a hard time tapping the holes, and needed to go to a bigger tap handle for more leverage.
-A larger tap handle was not at all needed. The holes are only 1/4-20 in 6061 aluminum, so this should have taken minimal effort, really, only finger effort. Should have been an obvious sign something was wrong. A much larger 3/8-16 thread cut into steel can still be done with only finger effort, and this person should have known this because this was one of their first projects in their first semester of machine shop.
-Cutting oil was not used. The piece had not been cleaned, and there was zero oil on the piece when I got it, so likely no lubricant was used. No oil means greater effort, higher load on the tool, higher wear on the tool, and worse finish on the threads.
-Likely the single biggest reason was I think, but haven't confirmed, that the wrong type of tap was used, it hit the bottom of the drilled hole, and when it did so and the person continued to turn the tap, it just tore off the threads it just created. My theory is backed by markings at the bottom of the drilled hole and someone else telling me what type of tap they took to go do the work.
Have the person work on something else that is easier, order threaded steel inserts, get a carbide spot drill from machine tech department and very slowly drill out the broken tap, also relocate 3 holes that were slightly off, drill and tap for threaded inserts, install inserts when they arrive in the mail.