2004 X5 4.4 (N62) Battery/Voltage issues

[TerraPhantm]

So this is actually about my neighbor's car, he asked me to take a look at it since it was giving transmission failsafe errors and occasionally not starting if left for a few days, and I'm kinda known as the BMW guy around here.

So first thing I checked was the error codes. Lots of low voltage errors stored in pretty much every module. And oddly one high voltage error (18v).

Next I voltage while the car was running. It was at 14v for the most part, but occasionally it would dip down to about 11v (battery voltage) while idling. Battery light never came on. Increasing load (defroster, heated seats, etc) seemed to keep the voltage steady at 14v. Voltage was also a steady 14v while driving. I know the alternator communicates with the DME on these newer DMEs, so I don't know if the altnernator briefly cutting out is by design, especially since the light never came on.

Next I charged up the battery until my charger indicated full (battery voltage was reading about 13 iirc) and left the car alone over the weekend. Came home and the battery was 6v. So I'm thinking parasitic drain. I charge up the battery again, hook up my multimeter, and checked the current after it goes into sleep mode. About 36mA which seems normal to me. Using the min/max mode it looks like it went as high as 92mA, which might be technically out of spec, but idk if that's enough to drain a battery so quickly. And the average is still 36mA.

So the battery doesn't appear to be original, but I guess it is probably dying/dead if it dropped to 6v, so I will ask him to replace the battery. Since there were some voltage errors stored too, I'm thinking the alternator could be bad (even though it wasn't really acting up in my possession). Is there anything else I should look for that's common on these X5s?