@mage_goo His statement has special significance, since his magic allows him to control his blood, and he has shown great regenerative powers in the past. As such, his statement is basically: "This damage is enough that I've lost control of my magic and I am unable to regenerate this much, so I will die."
As such, we have two options, when dealing with his statement:
1) Take him at his word, because we see the massive damage to his torso, so obviously he is about to die. (This is the option that most in this chat, at least among those mentioning it, seem to have chosen.)
2) Remember that demons use words to deceive humans in order to increase their chances of survival, and wonder if this might be a lie of his, to try to trick her, based on human perceptions of what looks like unavoidable death. If he is still capable of surviving, then the best thing he can do is to talk to her in order to try to find out her weakness, as well as convincing her that he can't survive, in hopes that she leaves to help Frieren, even mentioning Frieren being in danger to possibly prompt her to rush over there. (This is the option that Fern seems to have chosen, based on the fact that she makes sure to deal the finishing blow, and leave no doubt about whether he can regenerate. As such, and because his wound here isn't that much worse than the wound from chapter 17, I find myself choosing this option as well.)
Edit: Didn't realize until now that I had mistyped the name so it didn't send a proper notification.