Point out that their "love" is so profound that they will violate a basic tenet of Christianity and no longer honor their parents if they get "disfellowshipped". They cannot have any normal contact with them if they no longer are Jehovah's Witnesses. BUT if they lived in the same house, they could have normal contact!
This point of yours is profound in itself. I too am a parent in this condition. But when you think of it, if it's permissible to shun (which I don't believe it is) then what gives them the right to NOT SHUN with "... but it's okay if they live in the same house". Pure politics is what it is. They'd probably have the law all over them.
Same politics I learned when I went to Ministry School for Elders in the mid 70's. My question, as we learned the difference between disfellowshipping and disassociating was zilch. Why the two, I asked? Oh, brothers, we've learned to be cautious with our enemy. If we disfellowshipped someone who entered the military we'd be shut down in this country. We can't let that happen, can we? On the other hand, we can merely announce that so-and-so disassociated himself by joining. He did it! We didn't do it, understand?
I asked if that wasn't the "fear of man" that we were cautioned against and never got an answer.