I generally inform the client that they have a reporting requirement and offer to prepare the 1099s for them (for a fee, of course). If they decline my offer, then we have the discussion about deductibility and audits and losing audits and paying taxes and paying penalties and paying interest and possibly finding someone else to prepare their returns. I like to educate, and it is not my job to disallow deductions - that happens at audit by the IRS or the tax court. If it is a one off oversight, then we do what we can to make the return correct for filing.
Tom
Longview, TX