Question

Good evening!
The Rabbi has argued in several places that there is no prohibition of “do not place a stumbling block” with respect to an heretic (and presumably the same would apply to an apostate), since he is held accountable for the very fact of his denial.
I wanted to ask based on the words of the medieval authorities (Rishonim) in Bava Metzia 71b, where they ask why it is permitted to lend to an apostate with interest. After all, even if he is no longer considered “your brother,” you are still causing him to stumble by having him take interest. They answer that the borrower’s prohibition depends on the lender’s prohibition, and here it is permitted to lend to an apostate.
It seems from their words that there is a prohibition against causing an apostate to stumble?
And I would also like to ask from the well-known story about the Vilna Gaon, who saw an apostate eating without a blessing and said that he would also be held accountable for that?
Thank you very much!