My guess is that your client had previously taken $350 of the maximum $500 credit in an earlier year. Look for any entry for Credit claimed in ...

https://www.irs.gov/newsroom/energy-incentives-for-individuals-residential-property-updated-question...

Q. What improvements qualify for the residential energy property credit for homeowners? (updated April 27, 2021)

A. In 2018, 2019,  2020, and 2021, an individual may claim a credit for (1) 10% of the cost of qualified energy efficiency improvements and (2) the amount of the residential energy property expenditures paid or incurred by the taxpayer during the taxable year (subject to the overall credit limit of $500).