Yes, it could be recovered if he tried really hard, but not inadvertently. Unless you do a secure wipe of the drive you could still send it off to the data recovery guys. Is he seriously likely to do that?
Actually, it's not hard at all. There is an app that I downloaded the free trial of, it's called "Disk Drill," and I took my newly REFORMATTED Time-Machine backup (oh, by the way, I'm not 'yelling' when I capitalize letters... I'm just empherserzing). Anyway... I took my newly Erased and Formatted Time Machine Backup and ran Disk Drill, and pretty much EVERY FILE I ever had was still on there!
So, consider this post to be infomative -- Moderator? If you think this is an inappropriate post, PLEASE DELETE IT.... the reason it might be inappropriate is that some people
might use Disk Drill (or one of the bazillion utilities out there)
for inappropriate reasons and look through private stuff. I'm not making this post to recommend that. No. Rather, I'm making this post so y'all know how EASY it is to find files that everyone thinks is deleted!
How to do that? Just do a Secure erase. Me? I would be happy with a one-pass secure erase. That way, if I ever sell my beloved iMac (or get frustrated with it because SOME people on SOME forums say really, really dumb stuff), I would run a one-pass, or a three-pass secure erase to erase any banking info, etc.
[edit: added after sandwich]: Just for kicks, do this (you, or whoever's reading this), get a drive and erase it. First, load it up with some files -- some music, photos of grandma's casserole, rover balancing his bone on his nose, love letters to your teacher, whatever, THEN, erase your disk. Do it however, drag the files into trash, go into Disk Utility, partition / format it, whatever. Now, download Disk Drill and use Disk Drill on your disk.
Oh...my...goodness y'all are gonna be surprised! (And enlightened! Like I was).