The question of whether you should depends on your own ethics..."Michael"
obviously feels it's none of your concern. I disagree...as I've said before
in similar situations, if you could be sure the only person she'd kill was
herself, then it's an issue between her and her maker. But you can't be
sure of that, can you? She may kill a passenger (or more than one), and she
may kill people not in her airplane.

I had a similar siutation. I solo'd just before moving to Germany so
my radio skills were limited and XC time was zilch. My housemate in
Germany, myself and his pilot friend went up. We flew around southern
Germany and then to Salzburg. From my memory, things the guy did that
was clearly dangerous.

1. was going to take off without a checklist. he accidently found it
while taxiing.

2. I don't recall him doing a full pre-flight.
3. was quite lost going into Salzburg despite a Garmin 430
4. Through in full flaps flying into Salzburg Class C airport with
numerous planes barrelling down our ass. he could have easily thrown
in full flaps after crossing the numbers at 1000 AGL considering it
was a 9000 (?) foot runway.
5. he landed maybe 1 inch past the numbers and then proceeded
to roll down the runway to the FBO 4000 feet past the numbers
rather than landing very long.
6. on departure from Salzburg, couldn't follow basic radio commands,
"contact departure on xxx.xx, squawk YYYY"
7. got lost going back at night until Munich approach (not sure
of the names, BTW) said "Cessna XXXXX, if you are going to Augsburg,
wouldn't you want to turn by now."
8. MUC approach got on his ass for busting the class B (I'm guessing)
airspace.
9. (not in order) on approach into Rosenheim, I was in the back of
the 172 and remember treetops flying by at our height on
both sides so we were very damn low.

that is what I remember. I remember there were more.

Well my housemate and I recently flew from the bay area down to Santa
barbara. He commented that I'm a lot more precise flier with my
115 hours and half way through a IFR ticket than his friend with 250+
hours of VFR. I didn't want to bad mouth his friend until my old
housemate told me they went flying with 2 smallish women over the Alps
at 12000 feet. I wondered what the density altitude was and how
much AGL they were. Apparently there wasn't much. Then my old
housemate, granted he is NOT a pilot, said they had a serious close
call on final back at their home airport in Augsburg due to
language problems. I didn't get into it but it would not surprise
me whatsoever. This guy is simply scary, downright dangerous to
not only himself but his passengers and other airplanes/people.

After I returned to the US while finishing my PPL, I saw something
like that with a Mooney in the pattern. The guy left the pattern but
then returned 20 minutes later. I said to my CFI, "you watch him.
I fly." nevertheless the guy busted into the pattern at a controlled
Class D airport and then came within 200 feet of landing on someone,
then couldn't follow directions to climb to 1200 (pattern is at 800
and SFO Class B at 1500) and circle. I was in no immediate danger
but if he almost landed on me, I'd demand a write up of him.

IMHO, if this person is as bad as you desribe, I'd report them.

Gerald