He can't make the evidence of absence claim, but he can absolutely make the absence of evidence claim.
Perhaps in an “everyday language” way, but not in the technical, statistical sense.
In an underpowered statistical study, a claim that two experimental conditions did not differ are not persuasive.
Perhaps in an “everyday language” way, but not in the technical, statistical sense.
In an underpowered statistical study, a claim that two experimental conditions did not differ are not persuasive.