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I'm not able to answer two separate questions on this post. I will deal with the first question.
Do you know who provided him with the document that he signed?
So what you can do is ask the friend to show you what your father signed. You can say that your father didn't understand what he signed and that you are worried that he signed something that may harm him later.
But it is very possible that is was just a passport guarantor that your father did sign and if the friend says that is all it said then what you can do is send the friend a letter confirming that all your father signed was a passport guarantor and nothing more.
If he co-signed a loan then he could likely prove that he is a victim of fraud and have the agreement that he signed set aside.