Someone was davening Shachris when the tzibbur was davening musaf. They came to the bracha of machayeh hamasim the same time the tzibbur came to that point. What should be said together with the tzibbur when he’s in the middle of the silent shemoneh esrei. The nusach of Keser or of Nadishach?
Kedushah is something that is said communally, and therefore the Keter Kedushah will be said.
Although this is certainly non-ideal (to say the “wrong” kedushah for Shacharis; see Machzor Vitri p. 155, who explains some of the differences between the two), one should not say Na’aritzah when the congregation is saying Keter.