Having a 401(k) means your client has or had an employer.

Having only the 1099-R doesn't mean the client has no other financial activity.

And, again, "rollover" as in, he got the funds in between the distribution and the deposit? Is there also a Form 5498?

And you would need to determine if that rollover is Qualified, met all the requirements for not being taxable, the age of the client matters, are they still employed there?

Are you familiar with retirement funds and tax reporting?

Have you used the IRS resources, such as:

https://www.irs.gov/retirement-plans/plan-participant-employee/rollovers-of-retirement-plan-and-ira-...

https://www.irs.gov/taxtopics/tc413

Or web resources:

https://www.investopedia.com/articles/personal-finance/072215/401k-rollovers-tax-implications.asp

You are the only one who can do the due diligence to know if this client must file a tax return or not.