Ok, well I haven't actually taking my 4 year old yet, but I am planning to in March. From looking at the website it seems like there will be alot that he can do, however mine is a tough little booger. He loves TOT, Splash, Thunder, and is eating as much as he can so that he will grow tall enough for Space by our next visit. There are a few things that don't seem too scary though.
Has she already planned to go to Disney World and just thinking about adding US/IOA? If so I would suggest that she do Disney first and see how her kids are handling those rides (more kid friendly) and if they seem to be ok then she can go to US/IOA and buy her tickets at the front gate.
I hope this helps.
Dana