Is there something I must do after unknowingly signing the divorce papers of my friend's sister?

If your signature did not lead to injustice or corruption, then repentance to Allah is sufficient for you. However, if it did lead to injustice or corruption (for example, if your signature caused a woman to be divorced unjustly), then you must retract your statement to the relevant authorities and seek forgiveness from those who suffered injustice because of your signature.