If this is the place to ask the Attorney General a question about election law. Is it legal to contact a voter after they posted a ballot to ask them to edit a ballot, to change the preferences or to add office they didn't fill on the ballot.
Not the attorney general but here is my guess.
Currently, the Federal Electoral Act and the constitution specify the reasons why a ballot can be invalidated (among those, editing after 20 minutes or posting twice are included). However they say nothing about asking people to edit; so I don't think there is a good argument for a civil case.
As for a criminal case, back in the pre-reset days the law was a bit stricter on this regard and there was sort of a precedent involving LeBron Fitzgerald. However our current laws are a bit more relaxed. The relevant crime in Atlasia for something like this would be Electoral Intimidation:
(f) Electoral Intimidation. This offence shall be defined as the placing of undue pressure on a citizen of the Republic to cast a ballot in a particular way, to not cast a ballot at all, or to invalidate a ballot already cast via direct threats of revenge, blackmail, or retribution. In those cases in which undue pressure has been applied in the form of credible threats to disclose personal information, the offence shall be known as Grievous Electoral Intimidation.
So in other words, unless you can prove that a certain voter was blackmailed or intimidated, it seems to be legal.
There is however another crime that could be argued to have happened, that being Election Fraud:
(d) Electoral Fraud. This offence shall be defined as the manipulation of of the republic's electoral procedures and practices with the aim of fraudulently altering the results of a specific electoral contest.
You can argue that PMing someone to invalidate their vote is a manipulation of electoral practices; but that is not the greatest of interpretations. In fact under a very loose interpretation you could even argue that PMing people about the election at all should be illegal!
So I wonder what that crime is meant to prevent but whatever.
TL;DR: I am like 95% sure that PMing someone like that is legal