Short answer: yes, a lawyer might not be required to deal with one co-executor.
Long answer: Generally lawyers, like everyone else, are free to not talk to whomever they please unless they're obliged to talk to the person. And the only person a lawyer is obliged to talk to is their client or whomever their client directs them to talk to. In estates matters, the lawyer's client is the executor, not the estate, and the other executor may have no claim to the lawyer's services.
It is unclear to me how he could give advice that "goes against" the other executor. He is certainly entitled and in fact has a duty to give advice, and it may so happen that the other executor disapproves of that advice.
