you are inventing “rachmonus” on secular people, something that is forbidden by Torah for anyone who either acts secular (not keeping most mitzvos publicly), doesn’t keep shabbos publicly, or not believing in ikarei eminah.
such a person, regardless of tochucheh, hatrueh, or any o’nes (like tinik shenishbu), it is forbidden to have rachmunos on or to be dan lekaf zchis, rather they are chayev “mordin v’lo maalin”.
this is in contrast to the colloquial “tinik shenishbu” of karoyim (todays “modern orthodox”) where their kefira is specifically because of corrupted oral tradition, and exception the Rambam famously made from his otherwise normative red-line of the other 12 of 13 ikarim. Those, one would need to check if their mistaken psak stems from fraudulent “rabbis” that they follow innocently or from an ideology of rejection of Chazal. One checks by simply informing them and seeing if they indeed correct their mistaken ways or if they continue down the path of heresy. If they are the former, they deserve much rachmonus.