Client does not have to sign the 3115. From the instructions. Link. https://www.irs.gov/pub/irs-pdf/i3115.pdf

"Automatic change requests. Except if instructed differently, you must file Form 3115 under the automatic change procedures in duplicate as follows. • Attach the original Form 3115 to the filer's timely filed
(including extensions) federal income tax return for the year of change. The original Form 3115 attachment does not need to be signed"