Well that is why the affidavit has to be detailed. If you look at the template you should include information about how, where, and why the person knows you. They should be writing it. It is also better if they are USC or if you know them in a professional matter. If USCIS thinks is suspicious or not enough they will send you another RFE requesting more information and if they believe fraud has occurred they might and can request to interview you in person or that person. Ideally you would have the letter notarized. But, it doesn't have to be. So, make sure you are not lying or else is considered fraud.
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Sent 10/19->WAC accepted 10/24
I-797 arrived 10/27
Biometrics letter arrived 10/29 Biometrics done 11/20
EAD: February 7, 2013 SSN:Already Have
DL:Gonna take a month or two..haha.never bothered to learn..too scurred