Hi Bill! I'm assuming that the plant is the "orange" butterfly weed? If so, it's almost impossible to separate it because it has a long tap root. (...and it's almost as difficult to just transplant it.) Although Monarchs will feed on it, they generally don't lay eggs on this species of plant... usually just the common (roadside) milkweed.
Your friend's plant should have some cool seed pods on it right now (unless some critter ate them... the bunnies ate all mine.) You can keep the seeds over the winter, and then start them about 8-10 weeks before last frost, to get some seedlings. You can also try just getting some seeds and putting them in the ground before it snows... since it's a self-seeder... you may even get some plants "the easy way!"
Oh... one last comment about the big plant.... after first bloom, it will handle a fairly good "trimming"... then will often bloom again!