How To Write A Affidavit Letter For A Friend . The purpose of this letter is to work in favor of the immigrant in question by substantiating important aspects of their immigration case. Talk to the person immigrating to the u.s. Marriage Affidavit Template Free Printable Documents from londonmedarb.com You may also like affidavit of marriage examples. An affidavit letter of support is one of the support affidavits. The person for whom you write an affidavit should be someone you know very well.