If she was on heartworm preventatives, the company will pay for her treatment. If she was not, I strongly suggest you put both girls on one.
As for the treatment, the recommendation now is for them to take Heartgard for 2-4 months. This will kill the microfilaria (babies). My vet gives them Doxycycline for a month prior to treatment. That is given to kill a parasite that lives in the heartworms. It supposedly makes the treatment easier on them.
The best way to treat imo is in two treatments and they are a month apart. I am sure that is what your vet will recommend. It is a slower kill and much easier on them.
Here are a couple of links that have a lot of information about heartworms and treatments:
Heartworm Treatment - VeterinaryPartner.com - a VIN company! American Heartworm Society | Canine Heartworm
Her activity level must be kept to a bare minumum. I am sure your vet will explain that. Many people use strict crate rest. I only do that with a super hyper pup. If they are relatively quiet, I allow them to be out of the crate when I am at home. While out, I take no chances and crate them.
Good luck and let us know how little Sophie is doing!