May one interupet in the middle of Birkas Hamazon to answer amen to any berachah of Birkas Hamazon he hears from another. What is also the Halacha, for kaddish kedushah etc.
Birkas Hamazon should be treated like Shmone Esrei in this regard, and is forbidden to interrupt even for amen, kaddish, and kedusha.
Kaf Hachaim 183:43, Ben Ish Chai, Chukas 3, cf. Chazon Ish O:C 28:3,