Pike Livebearers, Belonesox, are super cool; think of them as Grass Pickerel from a different family. Much tougher to switch to non-living food IMHO; if your friend thinks that he is going to starve them for a week and then have them chowing down on pellets, he is in for a surprise. They need to be out-witted into eating non-living prey, and many aquarists don't seem to have the patience...or the guile...to manage it. Easy to breed but tough to get the babies out before they are all eaten. My idea of breeding fish consists of throwing adults into a tank or pond, and then removing a bunch of inch-long fry a few months later. You won't get many Belonesox fry if you follow that method; they have large broods, many dozens of fry from a big female, but very few survive even a couple of days in even the most densely-planted tanks. You manage to snag out a few if you are lucky enough to find them shortly after birth...and then you have a bunch of carnivorous, cannibalistic psycho-guppies that all want live food, which usually turns out to be...each other. You even need to watch out for the adult females, who get much bigger, eating their adult mates. But...as ferocious as they look...they are not really aggressive to similar- or larger-sized fish.
How about African Leaf Fish, Polycentropsis? I had one many years ago, I think P.abbreviata. They stay small (three inches +/-), have big mouths and big appetites. Predatory but not aggressive, i.e. an eater but not a fighter.
There is also the very similar Polycentrus from, I believe, South America. Never had one but they sound like pretty much the same deal, maybe even smaller in maximum size.
"Predatory" and "aggressive" are two completely different things. Predators eat anything that is an appropriate size, which can include some pretty bit prey for the large-mouthed jobs like these guys...but they aren't necessarily aggressive in their interactions with other fish that are too large to eat. On the other hand there are plenty of aggressive herbivores and omnivores (many cichlids, for example), i.e. fish that eat greens but just like to push other fish around for gits'n'shiggles, without actually eating them. I knew a Goramy once, owned by a friend, which would attack and eventually kill just about anything in the same tank...but never ate even the smallest guppy.
If your friend simply must have both...a carnivorous diet and a bad attitude...and wants to keep the thing in 30 gallons...I dunno what to say.
Backfromthedead
...I posted before I read your last comment. 
