OK! Don’t worry!
In fact, as I know, the deleted data (even deleted from the recycle bin) is not actually erased off from the computer immediately after the deletion. They are still there. As long as your bother acts carefully, he does have chances to restore them all back:
1. Do not use this computer any longer in case that any new data would rewrite the previous big files.
2. Take chances with a PC data recovery program to retrieve the deleted files. You may not know that there are plenty of data recovery freeware that could work much more efficiently than some paid ones, such as Recuva, TestDisk and iCare Data Recovery Free, etc.
He’d better select a right one according to your own conditions.
3. Save and back up these restored files on another drive or storage device in case of data loss.
Note: You should remind your brother to back up all his important data all the time in case of similar problems.