There are at least three ways attorney client privilege doesn't apply here:
First, arguably, is that there is no privilege regarding soliciting your attorney to commit a crime or to help with the commission of a crime. If McGahn believed he was asked to fire Mueller to obstruct justice, then there is no privilege. This is true in any attorney-client relationship.
Second, McGahn had effectively recused, or given Trump notice that he could no longer provide him legal advice with respect to the OSC investigation. According to McGahn, at the time Trump called to have him fire Mueller, he had already told Trump that he was a witness to what he believed was potentially obstruction of justice in the firing of Comey and that all matters related to the ongoing obstruction investigation should be directed through Trump's personal lawyers, not through McGahn. This effectively terminated any A-C relationship with respect to this issue, and resolved any mistake of fact regarding the existence of such a relationship. It put Trump on notice that he had no expectation of confidentiality with respect to statements made to McGahn about the OSC investigation. This is also significant to #3, below.
These were the two I had in mind when I initially posted that Trump wasn't McGahn's client. Depending on how you view it, that may have been a more accurate statement than Trump was never McGahn's client, because...
The most complicated of the three is the fact that McGahn was never Trump's personal lawyer. He was the White House Counsel, so he had an organizational client: The Office of the President. Privilege with respect to organizational clients is complicated because some organizations (like crime families) have tried to use it to hide evidence, and because organizations tend to have members, who may or may not have a privilege. It has developed that the privilege only applies to communications seeking legal advice. If you're asking for business (or in this case, political) advice or asking the lawyer to perform some act, there's no privilege. Obviously, this doesn't mean there can never be confidential communications between Trump and McGahn, there likely were, but Trump directing McGahn to take actions such as fire Mueller or produce false documents does not fall within the privilege.
Once McGahn has told Trump "I can no longer advise you on this matter, I'm a witness" Trump has no expectation that he can receive legal advice from McGahn, and all communications between he and McGahn regarding the OSC investigation should not be subject to attorney-client privilege.
There are other issues involving A-C P and organizational clients, but I didn't think any of them applied, here.
It's just more of the same nonsense that anybody who points out that Trump doesn't help bury Trump's **** must be incompetent or underhanded, and if you don't know how, you just make **** up.