Interest must be charged otherwise the lender will have to pay tax on a portion of the principal (loan amount) as it is paid back.
I don't have on hand the exact minimum interest rate required. It varies from year to year although I believe that a loan does not have to be adjustable rate to prevent this problem..
In addition, if no interest is charged and no loan documents are drawn up and signed and the borrower does not make payments regularly then the entire transaction may be regarded as a gift by the IRS and this may cause problems with inheritance taxes and gift taxes down the road. For an older lender, the gift may cause problems with being eligible for subsidized nursing home care.
But anyone person can give any other person up to $14k a year with no tax implication.