We have got a huge one of these in our pond, so correct advice about it needing to be wet. I know this isn't directly answering your question, but our plant is very attractive to the local snail population, so I would warn your sister to beware of attack if hers is direct in the ground, as yours is in a pot, a good smear of vaseline (or Poundland Petroleum jelly) should keep the nasty blighters at bay. Going back to your question, it may just be the shock of transplanting it that has caused the leaves to yellow, I'm sure it will pick up. I collected the seed from mine and now have lots of very healthy looking plants that I am hoping to sell to a local flower stall, all these plants survived being outside over winter and have, with all of the recent sunshine, burst into life many of them with flower stalks ready to open. Hope your one improves.