The examiners of a friend's PhD thesis required revisions which they needed to approve before they would accept the thesis. The requirements were fairly demanding but overall reasonable and helpfully explained, and my friend wishes to acknowledge the examiners' contribution in the final submission, in anticipation of their approval.
However, it seems to me that examiners are not supposed to be friends; and any implication that they are seems to suggest that the examination process was too cosy, or that the examiners were perhaps rather a push-over or even somewhat corrupt. Are such concerns well founded, or am I worrying unduly?
Peer-reviewers of academic research papers are not supposed to be authors' friends, and on many occasions are direct opposite. However, it is not uncommon to thank them in the final version of the paper, because their contribution often helps to improve the paper considerably.
In the same way, I think that it is OK to thank the examiners for their suggestions, which helped you to improve the presentation of your thesis.