Sure, you need invoices. But I don't think this is illegal with respect to the IRS. Though if your friend doesn't have $10 million in his LLC, then your friend

Sure, you need invoices. But I don't think this is illegal with respect to the IRS. Though if your friend doesn't have $10 million in his LLC, then your friend will become insolvent and that might be illegal. On the other hand, if you net your invoices regularly, then maybe the time spent being insolvent is short. IANAA.