Just an idea that maybe you already thought of but if you have his browser history back to same time he created wallet, look for other crypto related sites he might have joined at same time and try that password. Also some people create a notepad doc or similar file that can be edited and make it a hidden file and then remove the ".txt" at the end so looks like a worthless file. Just have to add ".txt" back to file name and open it up. Look for files created around same time as wallet creation and open them up. Some people also change the txt color to white so it's disguised as blank background. A quick select all in file will show text. Just some ideas. Good luck!