My friends father has a 3 year old rat terrier pure bred female. She got sick all of a sudden, no energy. She was told she has an enlarged heart and heart worms on top of that. The vet wants $500 to cure the heart worms. He said he had been using preventative medicines. Will the dog die from the enlarged heart though anyway in pain?

Does anyone know?

hiya
the dog will not necessarily die from an enlarged heart. It can be treated, although not cured. You can get medication for it which basically keeps everything ticking ok but it wont ever go away fully.
If I were you I would ask to have a proper chat with your vet. He should be pretty happy to explain it all to you and show you some diagrams.
Try not to get too worried. It isnt nice by any means, but there is medication.
I hope all goes well.

There are not many vets who know how to treat heart-worms. I will try to contact a friend whose dog suffered from the same problem which is not well known in this coutry apparently. I will try to find more details for you.