If you have a pair of convicts, then there's little point in mixing in other fish. Everything I can think of will be either too passive for breeding convicts or too aggressive for convicts without fry.
I suggest trying one convict and a large school of tiger barbs. Maybe a couple of bristlenose plecos. Be sure to include lots of ground cover: rocks, terracotta pots, driftwood, etc. Otherwise you won't get as much territory defense.
Although, now that I think about it, why does your friend want an aggressive tank? If he just wants to see fish behaving territorially, then he might want to get Mbuna cichlids (and a bunch of rocks). If he wants to have fish that try to attack him, then get a breeding pair of convicts, dempseys, or jewel cichlids. If he wants a predatory fish, well, most of those aren't actually that aggressive; they tend to be ambush predators and just lurk around.
In any case, you'll get a livelier tank if you give the fish things to hide around and defend. It sounds counter-intuitive, but providing ample hiding places tends to make fish feel more secure and more comfortable about being active in the open. Similarly, territorial fish like having landmarks that they can defend against encroachment.
But that's enough blathering. If you can tell us why your friend thinks that he wants an "aggressive" tank, then we can help you figure out what sort of fish to get.