Hi there,
I had a tonsillectomy last March so will hopefully be able to help you with any questions you or your niece might have! Im afraid the toast thing is still true - You're not allowed to go home until you've eaten it! They tell you to avoid doing the tempting thing of just eating soft foods as this impedes recovery somehow. It's apparently best to eat rough foods. I went in in the morning and stayed overnight. I think this is normal procedure. The first day is OK because they administer local anaesthetic to the area while you're in theatre so you end up waking up thinking "well thats not so bad at all!" but when that wears off it can be quite painful. I found that drinking cold drinks helped me. I was 21 when I had it done though and apparently the older you are when you have it, the worse it is! So if your niece is only young it (theoretically) shouldn't be so bad!
I hope that's been a little bit helpful. If you've got any other questions at all then let me know and I will do my best to give my opinion!