I don't think the concern should be for the "best" free data recovery program- it should be for any free recovery software. I have not seen any free (other than trial demos like Data Recovery II which will allow you to see what it could recover if you buy it) data recovery software at all.
If he has one that is formatted as FAT32 it can already be used with both. If he only has NTFS formatted drives you can install the free utilities MacFUSE and NTFS-3G on your Mac to write to it----BUT everytime you use your Mac at this point you are decreasing the chances of recovering everything since when you erased the files you told the OS that the space they formerly occupied is now free space for the system to use. Any that are overwritten by new data (such as installing new software) will not be able to be recovered.
well if you dont mind spending a couple dollars, i would just go to your local CompUSA or Best Buy and pick up Norton Disk Doctor for Macs. i have that on my computer and the recovery works pretty well, even things that have been deleted for 6+ months.