Forget net profit. What is GROSS income. 

Depreciation recapture is for allowed or allowable, not "taken", so YES they may have to pay capital gain on something they did not benefit from.

They should file to protect their SSN from fraudulent returns. They should file to stay in the system to get stimulus money. They should file because they have already wasted some of your time and you should get paid for filing the return.

When all else fails -  READ THE INSTRUCTIONS! - From the 1040 instructions, page 9 "**Gross income means all income you received in the form of money, goods, property, and services that isn't exempt from tax, including any income from sources outside the United States or from the sale of your main home (even if you can exclude part or all of it). Don’t include any social security benefits unless (a) you are married filing a separate return and you lived with your spouse at any time in 2019, or (b) one-half of your social security benefits plus your other gross income and any tax-exempt interest is more than $25,000 ($32,000 if married filing jointly).