there are a lot of different programs that can recover “deleted” data. Sometimes they work and sometimes they don’t. sometimes they can retrieve files, and the names are correct, but all the data inside is corrupted. sometimes it’s just the opposite…the files have gibberish filenames, but all of the data inside works. It really depends on how long it’s been since the stuff was deleted, hoe often teh drive has been used since then, the probability of overwriting that sector since the file was deleted, and whether or not your friend is using a reliable program.
the only way to get rid of your data for good is to do a low level format which not only deletes the data, but overwrites the entire drive with 0s. some people do 2 or 3 passes of “zeroing” the drive just to be safe.
you shouldn’t have given him your hard drive if you were worried about him finding data on there. or you should have requested your original hard drive back even if he gave you a new replacement drive.
also, this isn’t so much “spy” software as you state in your title but drive recovery software.
if you think your friend is out to spy on you then you might want to consider new friends!