I'm thinking when he was trying to select it, he may have s o m e h o w pressed "del" on his keyboard, but I don't think that deletes it permanentally. If it did, a pop up would come up asking to confirm the delete so I have no idea how this happened
This theory is the most reasonable. If you try to delete large folders in windows, it wont try to recycle it and instead just prompts you asking if you want to permanently delete it.
If he redirected his steam library to another folder, it doesn't seem far fetched he might had it in a place among other things he that was trying to delete, and just wasn't paying close enough attention when it asked for confirmation.
Nonetheless, he should have had time to cancel the deletion, although I'm not positive you can under those circumstances.
OP, If you seriously did delete your steam library; my condolences and you should download and use the tool Steve linked ASAP.