Well for the cost of a new GFCI I can buy at least 3-4 rolls of blue painters tape.
This is just wrong, would not trip on it’s own or with the suretest (highest % vd to date). The client was a first timer, had never heard of GFCI before.
Did you try to submerge your sure tester plug in water first. ha. ha.
Then you would know that the GFCI dosen’t work. Only joking. ha. ha.
Marcel 
Bruce,
I am glad you educated them…that GFCI needs to be replaced ASAP…