He would have to report the form 1099-B information as it is listed on the form. When he goes to enter it in TurboTax, he will see this screen:

He needs to click on the "I'll enter additional info on my own" tab and that will allow him to enter a new cost basis for the sale, equal to the one listed on the 1099-B form, plus your share of the gain on sale. That will result in an adjustment on his tax return to reduce his gain for your share of it.

On your tax return, you will go to the "Investment Income" section and find "Stocks, Mutual funds, Bonds, Other" and indicate that you do not have a form 1099-B, and you will see this screen:

Choose the "Stock" option and enter your gain as the sale amount and leave the basis empty.

Thank you so much for that explanation, @ThomasM125 -- very helpful.

One question: You said I should leave the cost basis for the stock empty. Could you explain why? I did pay for the stock when I acquired it originally, so I would think I should report that cost basis so I'm not paying taxes on the entire proceeds.

Thanks again!

If you have the basis in the 1099-B, then you can use that. If the 1099-B doesn't have the basis recorded, then use the basis you have and make sure you keep your records.

You should enter the cost of the securities as cost basis. However, you do this as an adjustment entry to the cost basis. If the form 1099-B does not list a cost basis, you enter it as -0-. However, you then click on the "I'll enter additional information on my own" and enter the cost basis on the next screen. That way, the IRS will see the form 1099-B entries as they are reported to you, but then with an adjustment to bring the reported basis in line with the actual basis.