The question isn’t whether a Talmidei of one of the mattirim can carry, the point we’re discussing is whether a talmidei of one of the osrim can lend a book (good example) to a talmid of a mattir.
If I were trying to make up a new issur, your car analogy would work. I’m not, though; I’m saying what I think is a reasonable sevara that since I hold something is assur, I also hold aiding you to do it is lifnei iver, an issur which certainly exists.
Let’s put our question another way: would a Beis Hillel talmid suggest a tzaras habas as a shidduch for a Bais Shammai talmid?
I think either way is reasonable, so I’d like to see mareh m’komos, not just, “you’re wrong because the Shulchan Aruch doesn’t talk about your case”.