Are you still looking for help with this? if yes, a couple of questions.
Is the customer field on the testimonial a Relationship field or a Post Object? Since I’m assuming that there can only be a single customer related to a testimonial it makes more sense to me that it would be a Post Object field which would return a single post object instead of an array.
If this is not the case can you do a dump of
$testimonialCustomer right after you get the field? Something like this would work`
'; print_r($testimonialCustomer); echo '
then post the results.
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