Parasite eggs can be very hard to kill and can live through "hell"... I know this personally from my experience with aquarium fish. I have successfully gotten rid of parasites like cappilaria and ich before but there are some that are relentless. Flukes would be an example of those, the trick in killing parasites is usually knowing the parasites life cycle and when to come back with the second, third or fourth dose. Tape worms are nasty but they are on the easier to get rid of list, follow your vets instructions to the T and you should be fine. I would recommend getting some of your other snakes poop tested just in case. When I was heavy into Discus I had a microscope on hand and learned how to look for eggs in poop, very interesting stuff. Snakes are a little hardier though and no need to be so picky. You definitely want to get rid of those tapeworms though, have you by any chance ever thought of the infected snake as a slow grower? Just wondering because with fish tapeworms really slow down their growth. If thats the case then you should see some faster growing from this animal once you get rid of the worms.