By the same token, I don’t see how an accused can make a fully-informed decision if the cops are lying to him. But the accused doesn’t have a particular right to be told only the truth at that stage of the criminal process.
The police are required to make sure the accused knows his constitutional rights. Beyond that, they can lie. The fact that a defense attorney is in the picture doesn’t change that basic equation, as far as I can tell.
Of course, the prudent practice for the attorney is to advise his client to say nothing at all. Whether the police lie or not is of no moment in that circumstance, because the accused will simply remain silent.
If there’s a deal to be struck, it will be with the Commonwealth Attorney anyway, not the police. Seldom is there benefit in speaking to the police.