“Here, they are flatly wrong, and their drawback is not knowing the metziyus. ”
Whilst I did not see the full responsum inside, I agree to a point. And as I have commented before, a blanket psak like this is above their head (K’vodum b’Mokomum..) and needs to be addressed by true Gadolie Hador via meeting with experts in the field and all involved. You can not just take a Rambam and say some pshat and pasken for a Rabim like that without truly investigating the facts and hearing all sides before deciding.
E.g. As I have mentioned, there is the psak that you are permitted to be Mechalel Shabbos to save one from Shmad, even though we pasken אין אומרים לאדם חטא בשביל שיזכה חברך and we never say חטא בשביל שיזכה חברך when he/she is a status of פשיעה, but there are exceptions when it comes to total leaving Yehadus and there is hope of saving a soul (שו”ע סי’ ש”ו- י”ד – see משנ”ב סי’ ש”ו ס”ק נ”ז ).
If kiruv has a chance of return, why shouldn’t it override the Issur Kal of (I am not even sure the above situation is even a Rabonon, especially the reasoning of the Rambam :ומחשבה של אפיקורוס לעבודת כוכבים) “associating with a Mumar” (le’Tayavin) ?!
Note – In the Rambam’s times, Mumrim had no record of returning. If one left rarely did they return.