You don't want her to wear Vaporflys on a cross country course because they have no traction and a thick sole, so there's potential to roll an ankle on uneven ground. They are designed for road racing, but some people like them on the track as well. So it would be a decent road/track combo shoe.
For a cross country/track combo, look for a lightweight cross country shoe with removable spikes. Not all cross country courses are spike-friendly, but they are an advantage if it's muddy or soft ground. Plus she can wear the spikes on the track.
It can be hard to find deals on cross country spikes right now because it's right before the season, but I often see them discounted at stores like Ross, TJ Maxx, RunningWarehouse.com, etc. Even full retail price, there are decent ones for less than trainers cost.