Rooting = voided warranty, correct?
Even if that presumably is the case, that still doesn't make him dishonest. Should he have gone in there and said "Look my screen is busted, but I'm rooted, so my warranty is voided, right?"
You would have to be an idiot to:
a) Not try to get your defective hardware replaced under warranty
b) Voluntarily tell VZW your phone was rooted, thus potentially preventing said hardware from being replaced.
Even if he had told them he was rooted, if they had any decency they would have replaced the phone anyway since it is not a software related problem. The real question is why does it bother you (as someone who has rooted their phone) that someone who was rooted got a replacement under warranty?