re: Update in OP; Treating young lab for heart worms

If you can prove you had her on preventative her entire life and she still got HW I'd contact the maker of the preventative. Myself and a friend have both been through it with Heartguard, my friend had to treat his dog twice. heartguard paid for both of his treatments and only the medicine for mine (since I started him too late on the preventative, so they said). My dog was about the age of yours when treated and he made a full recovery. If the HW infestation is not bad the treatment shouldn't be a huge deal.