The IRS is not a court of law - the burden of proof is on you. As such, I would make sure there were some record of how much your friend is getting paid, otherwise they will assess the $100 as your earnings. If you end up paying your friend over $600 in a calendar year then you will also need to submit a W2 for them.
http://www.irs.gov/uac/Form-W-2,-Wage-and-Tax-Statement