I have never owned a Ducati, but my friends have
(Like elo pointed out)
Anyway, it's true that they are unreliable, but nowadays this is limited to small things, eg: burning bulbs, small leaks, etc. I haven't heard of major engine problems lately.
Maintenance is also high, mainly because of the desmo and the fact that two heads need to be opened like in any other Vtwin for valve adjustments etc. Clutches are also a bit problematic in town.
Maintenance is crucial to Ducatis, if it's done properly they can last for ever, if not they can break down a lot.
I rode with a 1976 SS 750 moded by NCR to racing spec and it was running great, and I ve also pushed for 5 kms (using the peg technique) an almost new 900SS.
The Monster is a great bike though, more user friendly than any other Ducati, a fun bike really.
I would rather look at the 900 though, the 2 valve with the 900SS engine, not the S4 as it's too expensive, unless your friend has the money and is crazy enough for this bike.
The 600 is very slow, and the 750 is not much better. The 900 has enough power for good fun and the price is very similar to the 750.
The S4 is a basically a naked bike with the 916 engine!
I hope your friend finds what he wants