If she has a freshwater tank, there is
Kissing Gourami... but you would have to make sure the fish already in the tank are compatible. Also. Make sure her tank is large enough and has room...
If you decide on something, I'd suggest asking her before you purchase anything... she might not want the fish/coral, have room for it, or it might not be compatible with her tank.
Other than the
Kissing Gourami (freshwater), there might be some coral that is "heart shaped" or something...
Such as this brain coral...
Though that's not how the actual coral is... it's just the shape it presently has.
If she has a reef tank, some beautiful, easy to take care of coral that you probably can't go wrong with surprising her with would be xenia, polyps, mushrooms, or starpolyps (or something.. brainfart.... ><)
I'd go to a saltwater fish store and check out some of the corals they have there, write down their name, and research them... make sure they are compatible with what she has in her tank... if she has corals there's many that are compatible with almost anything. If she does not have corals she might have improper lights or a coral eating fish/starfish.
I'd be careful when buying her a fish... my brother's girlfriend bought me a fish once and it was huge and I felt horrible having to tell her to take it back to the store...