A few options:
1. If she only needs to move data one way, from PC to mac, then NTFS is fine. Mac can read NTFS but not write it.
2. Use 3rd party apps to let Mac read/write NTFS, or PC to read/write HFS
3. Format drive in exFAT or FAT32. FAT32 has file and drive size limits, so depending on size of external drive, choose the best one.