First, this is a ruling in a State Appellate Court; it has NO standing outside of WI, and no foreseeable impact on Federal law.
The scope and jurisdiction of the ruling notwithstanding, there is currently no way for an illegal alien to legally purchase a firearm from an FFL holder. They could buy from a private seller (though to do so would still be a crime as they, under current Federal law, are ineligible).
Sistema brings up a valid point, though I would argue that they SHOULDN"T be walking among us, they do so only because of the failure of government to enforce the law. We have a serious problem when about 3% of the population is here illegally....
Their immigration "status" is the result of a criminal action, period. WHY they committed that crime is irrelevant, an excuse. Every day that they remain, they choose to perpetuate that crime. I have little sympathy for whatever circumstance an illegal may find themselves in while living a crime on a daily basis. They are 'victimized' as a result of their own criminal actions, their own choices.
I could also argue that a criminal, actively committing a crime, has no right to possess arms during the commission of a crime. Most current law supports this...
If one believes that 2A right is an absolute human right in this country, then needs to be absolute, it can NEVER, under any circumstance, be stripped away by government... to include when one is in prison, or on a commercial airline, or in a courthouse, or in or on privately owned, publicly accessible property, certainly while on government property, or when under the influence of drugs or alcohol.... That's an "absolutist" perspective; we all know that it will NEVER happen...