Whether a state has made it "legal" or not, the courts should throw it out as inherently fraudulent.
Any election tainted with it should be regarded as null and void.
That's not the point. The Federal courts have no authority in state elections except when it comes to voting rights and civil rights of voters.
If ballot harvesting is illegal in Pennsylvania but they allow it anyway, the only recourse to it is in Pennsylvania. Another state doesn't have the legal standing to object to this. This is especially true if the objecting state allows ballot harvesting itself.
If you get a Federal court involved in a case where two states have different election processes and one is challenging the legitimacy of the other, the end result (if a Federal court hears the case) is going to be a court ruling that states must all have the same voting standards across the entire country. And guess what that means, folks ... the least restrictive voting standards are going to be applied across the entire country. That means ballot harvesting, multi-week early voting, drop boxes on every corner, etc. are going to become the national standard.