Do you pause and wait for him to finish and then resume your amidah?
Do you ignore the kadish and simply continue?
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This is a machlokess between Rashi and Tosafos (do you listen to kedusha mishum shomea ke'oneh or not).
The ordinary hesber is, if you say shomea ke'oneh is as if you spoke this would comprise a heksek, if shomea ke'oneh means you are yotze with the other person's amirah then you have not made a hefsek.