I don't really have any tips as DC did most of it themselves. The visa application entails a trip to the US embassy, which can be an early start and expensive to get into London on time.
DC took the mandatory Gallagher health insurance (think there was the choice of cover levels so they did the max).
They also took out travel insurance (to cover the usual stuff like flights and possessions) through Go Walkabout - give them a ring to make sure she picks the correct product. It automatically covers the 1 month travel period at the end of the J1 visa and can include any travel beforehand eg going out a few days before the programme starts.
Mobile phone, DC bought an Airolo sim to use.
They need to supply their own stuff for the apartment (bedding, towels,kitchen stuff). They shared Ubers to go to Target or Walmart to buy stuff. DC took some spare bedding from home, vacuum packed.
Florida is expensive, we had a list of toiletries to take out when we visited, as for example a deodorant that's £1.50 here was $6!
Are you planning to go out?