Yes, the pill will kill the live tapeworms and she will "evacuate" them. It will take a few days to do it so it may be a week before her poop is firm. If it isn't or if you see the signs of tapesworms (something that like looks like rice) next week you should call the vet for another dose.

If you aren't sure what to look for do a google search on tapeworms in cats and you should be able to find pictures. Segments of the tapeworms come out and will look like little white spaghetti when fresh and then it will dry up to look like a piece of rice. Rather gross depending on where you find it. Luckily it's easily treated and the one dose may be all Jenna needs. If she's been gassy you should notice a reduction in that too.