She is talking about he won't be able to remain human as they can't be mixed with the other world.
assuming the white blob we saw at the end of this chapter is his soul/humanity (?) i think it still is in the human world.
if you know "Stranger Things" i think the upside down world is similar to what she's talking about human world and another world. So what she's trying to say, if yoshiki is still being mixed up with hikaru, he might drag him to that 'another world' or rather might become 'impurity' as well.
Oooh now I understand. Thank you guys for all the replies! That was really helpful <3
"Tsujinaka-kun... He'll no longer be able to return to the human world" What does she mean by that? Does that means Yoshiki's soul is actually no longer in the human world?