Try bruteforce it. I dont think people will forgot his password and not remember even a clue. Try to use your daily password. Or ia the wallet ypu found it somewhere ? Then you must bruteforced it.
That's possible that someone will forget his passwords even if they're just normal combinations of numbers and letters. But the solution to this is to force himself to remember anything that could be his password. No one can help him on this one but just to brute force it with any password that he might have using. Because if there's no way for him to remember any password that he have used to this wallet, no help will arrive on him. He may have a clue if he tries to remember it but then, just keep on trying or just give it up.