legally you're completely in the wrong, even as far as BS7671 is concerned.
if it was a new installation it should have been safely isolated and locked off by you, with one key only - held by you! and the installation never energised unless it was done by you, after comleting the tests and the relevant paperwork.
if it was a periodic inspection and you came across an immediately dangerous situation, your first action should always be to stop testing and inform the client without delay and strongly reccomend the installation be made safe by means of isolation.
it's not up to you to turn someones power off (unless it is included in the extent and limitations of a PIR), only advise them as a competant person