Although I am neither a US-citizen nor a practitioner of jurisprudence (crimothy or otherwise), I will stick my neck out and argue that a charge of involuntary manslaughter or negligent homocide is incorrect because it implies the absence of criminal intent. Instead I would suggest Lester is guilty of aiding and abetting murder.
My reasoning is that Lester's appears to possess a mens rea that falls within the purview of the doctrine of accomplice liability:
The classic model of accomplice liability requires that an accomplice
intends to promote or facilitate the commission of an offense and,
consequently with this intent, aids the principal actor. This intent
requirement ensures that the accomplice has a stake in the principal's
acts; in effect, the accomplice makes the acts his or her own. Since
the accomplice's conscious objective is that the underlying crime be
committed, and thus aids in its commission, it is fair to hold the
accomplice as criminally culpable as the principal.
I'm sure that Lester would prefer that nobody be murdered. However, his actions and omissions facilitate a murder. By having Linda go the office, Lester at the very least demonstrates an awareness that there is a threat to safety. If someone has to die, he most certainly would prefer that it not be him. This might be reminiscent of negligent homicide were we to end it there.
Far more telling of Lester's complicity is that he has his wife don his parka while blind to the threat that doing so poses. We note that he has Linda place the hood over her face. This makes him an accomplice to murder because his actions cause Lorne to murder "Lester" — or more accurately -- someone bearing his likeness. This murder is far less likely to have been carried out had Lorne not misidentified her as Lester. That Lester is the intended victim of this crime need not render moot his complicity in the commission of this crime.