Samak left a private voting booth to check a list of candidates' names.
Sodsri said the Election Act does not permit voters to remove ballots from polling stations.
Samak had sought permission from officials before leaving the booth, she said.
Samak and wife Khunying Surat voted at the Moo Ban Chokchai polling station.
Samak checked his name on the list of registered voters, took his ballot paper into a booth, but then exited it to check candidates' numbers.
He said he forgot the number of his preferred candidate.
The station's director, Chuachan Taepongsorat, said Samak did not break the law because he did not remove the ballot paper from the booth.