What the title says. My Canadian friend got arrested for assault and death threats, but was later acquitted of all charges in 2017. Would he have any issues entering the USA?
An arrest, without actually being charged, does not make a person inadmissible to the US. From Canadians Requiring Visas:
Furthermore, Canadians who have been deported or removed from the United States, or Canadians with a criminal record, including for driving under the influence, must satisfy other requirements to gain entry to the United States.
If your friend does not have a criminal record in Canada, then there is no problem with admissibility on that point.