There is no required format except that each affidavit must be notarized.
Mr. Shusterman's (former INS Trial Attorney, 1976-82) response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
I would suggest including their full names, date of birth, place of birth, address and contact information in the affidavit in addition to the details of how they know each other and their attestation to the details of the event.