He received the password via SMS on his cell phone, but he no longer has the number and has never seen it.
It's best to contact the cell provider or the telecom that he has registered with that number. He can ask an attorney for him to request that number back to him for important filing like this if ever the provider denies his request.
There are too many tutorials about how to unlock PDF with a password without using one. But I doubt that none of them will actually work but instead will leave you some malware/virus as they require to download a software that's unknown.
I have already advised my client to contact a lawyer, I have some software to crack the pdf password, the problem is that the password is 8 characters too long, the processing power is unfeasible.
Find more in some other tech-focused forums maybe there have been folks that have done this in the past. Because it seems that you'll not get any help from anybody here even I, I don't know what to say further.
So, I'd just say good luck.