It was obvious to me that what your client did was borrow at a low interest rate and invest the loan proceeds for a higher return (but still safe, at least until the Treasury defaults because it exceeded  the debt limit).  And yes, this is how banks make their money.  Usually they file an 1120 and not a Schedule C.  But if you want to call it a business and put it on Schedule C, IRS has a code for that:  522100.  "Credit Intermediation."  I wouldn't rule it out, just because he just started doing it.  There's a first time for everything.  However, it may come under the "day trader" rules, where interest goes on Schedule B (like sales go on Schedule D) and only the expenses go on Schedule C.  In my opinion, I have no opinion.