There are plenty of big fish that aren't aggressive and would do great with kissing gouramies - take, for example, clown loaches, bala sharks (though they realy need at least a 125 gallon), tinfoil barbs and other large barbs, giant scissortails and other large rasboras, medium-sized to large plecs, many different catfish species and some of the more peaceful cichlids like the shy keyholes, many dwarfs and pulcher/kribs (though these aren't as big). Firemouths might also work but I've never tried this combination. A large school of some kind of medium-sized barb, tetra, danio or rasbora (such as clown rasboras, scissortails, rosy barbs, giant danios etc) can be very useful for enhancing the over-all look of the tank and, in a large group, can be just as impressive as a single large fish.
If what your friend is after is a big cichlid tank, kissing gouramies are not the best choice. They can handle aggression but, in time, they will be overwhelmed as the cichlids mature. If you go for cichlids instead, I'd suggest going for africans - malawi rift lake mbuna are perfect if you want color and activity and they are also quite hardy and easy to care for provided you get the water chemistry (they need hard, alkaline water) and feeding (most are herbivorous and can get bloat from fatty foods like worms) right.
Alternatively, maybe look into some of the predatory oddball fish. There may be some (those that don't get too big) that you could keep with kissing gouramies and some cichlids.
