The populist is simply playing his most obvious card, it is a good distraction from his own failings. If the quarantine hotel/system worked appropriately in WA it wouldn't matter where this person had been, when, or for how long.
But I will play along with McGowan (or McClown has he has been annointed by some here, with justification) - what did the person concerned tell the Commonwealth government the purpose of their trip was?
If they said "attend a family wedding" then yes I would lay some blame at the foot of the feds.
If this person said "family member on death's door" and then turned out they attended a wedding - then that is obviously an entirely different story.