Tekeny’s death is an important character beat for Nerys and important in the grand scheme of the war. On the other hand, it is much less important for Julian, as he has no such issues to get past, and at this point it would just be adding one more bit of hell for mid-S5 to put him through without providing any character development for Julian. Added to which, a) Julian is a proven medical genius and b) Tekeny comes to the station a full month earlier here, before he’s even had his Yarim Fel diagnosis. I tend to imagine that Ties of Blood and Water takes place over the course of at least two weeks, and that the disease tends to move fairly rapidly after a certain point, so actually, if Julian can get him in for a medical check-up, I have got enough wiggle room for Tekeny to receive effective treatment before his condition becomes serious enough that it cannot be cured.
That said, my undying love for the Cardassian dissident movement, and especially Tekeny Ghemor, is a bias of mine, so, I have come here to put it to a vote: do I kill him off or let him live?