Spain and France both can limit the number of reservations for Eurail pass holders on fast trains. For a quick route from Barcelona to Rome, we would have recommended flying, if asked in advance, versus a 20-hour train ride. But it's possible that the Eurail Aid office can help her find slower trains that don't require reservations or less popular departure times that still have reservations available, especially if breaking the trip over a couple of days. Of course, this is high season along the south coast of France and not many trains per day cover the whole distance to Nice or the Italian border.
Part of Eurail's mobile app is intended to help identify which trains don't require reservations, but I don't believe that it sells reservations or shows availability.
The situation will not be the same for trains in Italy, Croatia, or Greece (although Greece has very few trains) as these don't limit the places for pass holders and some don't require reservations. However, the separate countries can be more easily connected by flights, depending on the desired route and the time available. The pass does still cover ferries from Ancona or Bari to Patras, Greece, plus a port fee and sleeper fee (if desired).