Luckily, I have good access to a lawyer on a daily basis 
This is not the lawyers area, so dont take it as absolutely correct! You should consult your personal lawyer before making any decisions about this topic
I think it really depends on what the language of the non-disclosure agreement says. Obviously this isnât my area of expertise. What I can find is that some non-disclosure agreements say they do take precedence over all other contacts between the parties. Others are incorporated by reference. There are also several types of non-disclosures. For instance, you may have a prospective client sign a non-disclosure before negotiations so that should negotiations break down they do not disclosure your pricing structure, or you do not disclose their design concepts.
A contract for services, is likely to have boilerplate language which says something like the parties agree to maintain confidential information, trade secrets, etc. received by the other partyâŚdisclosure of which constitutes a breach of the contract. In those instances, what constitutes âconfidential informationâ should be defined.
As the guy in the post says, his client doesnât ever sign the PO. Not all contracts need to be in writing to be enforceable. In Michigan, you can have an oral contract so long as it can be performed within one year. But even an oral contract would be subject to common law.