You can get a Verification of Non-filing Letter from the IRS by filing Form 4506-T. You might also be able to get it by calling the automated transcript request line at 800-908-9946. The person who has not filed has to request the letter themselves. You cannot request a letter for someone else.
You can download Form 4506-T, including the instructions, from the following link.
https://www.irs.gov/pub/irs-pdf/f4506t.pdf
